'nova resize-revert' does not handle the deleted flavor

'nova resize-revert' does not handle the deleted flavor,
and an exception happens in the case.

* How to reproduce the problem on DevStack:
  $ nova flavor-create sample 10 512 0 2
  $ nova boot --image cirros-0.3.0-x86_64-uec --flavor sample test01
  $ nova resize test01 m1.tiny
  $ nova flavor-delete 10
  $ nova resize-revert test01
  ERROR: The server could not comply with the request since it is either malformed or otherwise incorrect. (HTTP 400) (Request-ID: req-b0d3e016-9608-4a87-a0cc-44dfe00b25a1)
  $

I feel it is better that 'nova resize-revert' prints a error reason like
"Previous flavor could not be found".

* The contents of nova.log
---
2012-12-15 01:10:07 22693 ERROR nova.manager [-] Error during ComputeManager.update_available_resource: Instance type 6 could not be found.
2012-12-15 01:10:07 22693 TRACE nova.manager Traceback (most recent call last):
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/manager.py", line 175, in periodic_tasks
2012-12-15 01:10:07 22693 TRACE nova.manager task(self, context)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/manager.py", line 3257, in update_available_resource2012-12-15 01:10:07 22693 TRACE nova.manager rt.update_available_resource(context)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/openstack/common/lockutils.py", line 228, in inner
2012-12-15 01:10:07 22693 TRACE nova.manager retval = f(*args, **kwargs)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/resource_tracker.py", line 269, in update_available_resource
2012-12-15 01:10:07 22693 TRACE nova.manager self._update_usage_from_migrations(resources, migrations)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/resource_tracker.py", line 452, in _update_usage_from_migrations
2012-12-15 01:10:07 22693 TRACE nova.manager self._update_usage_from_migration(resources, migration)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/resource_tracker.py", line 416, in _update_usage_from_migration
2012-12-15 01:10:07 22693 TRACE nova.manager instance_type = instance_types.get_instance_type(itype)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/instance_types.py", line 157, in get_instance_type
2012-12-15 01:10:07 22693 TRACE nova.manager return db.instance_type_get(ctxt, instance_type_id)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/db/api.py", line 1271, in instance_type_get
2012-12-15 01:10:07 22693 TRACE nova.manager return IMPL.instance_type_get(context, id)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/db/sqlalchemy/api.py", line 127, in wrapper
2012-12-15 01:10:07 22693 TRACE nova.manager return f(*args, **kwargs)
2012-12-15 01:10:07 22693 TRACE nova.manager File "/opt/stack/nova/nova/db/sqlalchemy/api.py", line 3641, in instance_type_get
2012-12-15 01:10:07 22693 TRACE nova.manager raise exception.InstanceTypeNotFound(instance_type_id=id)
2012-12-15 01:10:07 22693 TRACE nova.manager InstanceTypeNotFound: Instance type 6 could not be found.
2012-12-15 01:10:07 22693 TRACE nova.manager
[..]
2012-12-15 01:11:31 22693 AUDIT nova.compute.resource_tracker [-] Updating from migration 58d2088e-a771-4cd0-9964-8d83ce260dc3
2012-12-15 01:11:31 22693 ERROR nova.manager [-] Error during ComputeManager.update_available_resource: Instance type 6 could not be found.
2012-12-15 01:11:31 22693 TRACE nova.manager Traceback (most recent call last):
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/manager.py", line 175, in periodic_tasks
2012-12-15 01:11:31 22693 TRACE nova.manager task(self, context)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/manager.py", line 3257, in update_available_resource
2012-12-15 01:11:31 22693 TRACE nova.manager rt.update_available_resource(context)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/openstack/common/lockutils.py", line 228, in inner
2012-12-15 01:11:31 22693 TRACE nova.manager retval = f(*args, **kwargs)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/resource_tracker.py", line 269, in update_available_resource
2012-12-15 01:11:31 22693 TRACE nova.manager self._update_usage_from_migrations(resources, migrations)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/resource_tracker.py", line 452, in _update_usage_from_migrations
2012-12-15 01:11:31 22693 TRACE nova.manager self._update_usage_from_migration(resources, migration)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/resource_tracker.py", line 416, in _update_usage_from_migration
2012-12-15 01:11:31 22693 TRACE nova.manager instance_type = instance_types.get_instance_type(itype)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/compute/instance_types.py", line 157, in get_instance_type
2012-12-15 01:11:31 22693 TRACE nova.manager return db.instance_type_get(ctxt, instance_type_id)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/db/api.py", line 1271, in instance_type_get
2012-12-15 01:11:31 22693 TRACE nova.manager return IMPL.instance_type_get(context, id)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/db/sqlalchemy/api.py", line 127, in wrapper
2012-12-15 01:11:31 22693 TRACE nova.manager return f(*args, **kwargs)
2012-12-15 01:11:31 22693 TRACE nova.manager File "/opt/stack/nova/nova/db/sqlalchemy/api.py", line 3641, in instance_type_get
2012-12-15 01:11:31 22693 TRACE nova.manager raise exception.InstanceTypeNotFound(instance_type_id=id)
2012-12-15 01:11:31 22693 TRACE nova.manager InstanceTypeNotFound: Instance type 6 could not be found.
2012-12-15 01:11:31 22693 TRACE nova.manager