human-readable error messages on parser failures, not python stacktraces

I introduced a bug in my test gadget.yaml, and ubuntu-image told :

$ ./ubuntu-image -w workdir -d --resume
DEBUG:ubuntu-image:-> [ 2] load_gadget_yaml
ERROR:ubuntu-image:uncaught exception in state machine step: [2] load_gadget_yaml
Traceback (most recent call last):
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/helpers.py", line 85, in inner
    return func(*args, **kws)
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/parser.py", line 233, in parse
    validated = GadgetYAML(yaml)
  File "/usr/lib/python3/dist-packages/voluptuous/schema_builder.py", line 192, in __call__
    return self._compiled([], data)
  File "/usr/lib/python3/dist-packages/voluptuous/schema_builder.py", line 486, in validate_dict
    return base_validate(path, iteritems(data), out)
  File "/usr/lib/python3/dist-packages/voluptuous/schema_builder.py", line 324, in validate_mapping
    raise er.MultipleInvalid(errors)
voluptuous.error.MultipleInvalid: required key not provided @ data['volumes']['pc']['structure'][0]['type']

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/state.py", line 68, in __next__
    step()
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/builder.py", line 144, in load_gadget_yaml
    self.gadget = parse_yaml(fp)
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/helpers.py", line 87, in inner
    raise new_exc from exception
ValueError
ERROR:ubuntu-image:Crash in state machine
Traceback (most recent call last):
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/helpers.py", line 85, in inner
    return func(*args, **kws)
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/parser.py", line 233, in parse
    validated = GadgetYAML(yaml)
  File "/usr/lib/python3/dist-packages/voluptuous/schema_builder.py", line 192, in __call__
    return self._compiled([], data)
  File "/usr/lib/python3/dist-packages/voluptuous/schema_builder.py", line 486, in validate_dict
    return base_validate(path, iteritems(data), out)
  File "/usr/lib/python3/dist-packages/voluptuous/schema_builder.py", line 324, in validate_mapping
    raise er.MultipleInvalid(errors)
voluptuous.error.MultipleInvalid: required key not provided @ data['volumes']['pc']['structure'][0]['type']

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/__main__.py", line 110, in main
    list(state_machine)
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/state.py", line 68, in __next__
    step()
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/builder.py", line 144, in load_gadget_yaml
    self.gadget = parse_yaml(fp)
  File "/home/vorlon/devel/canonical/ubuntu-image/ubuntu_image/helpers.py", line 87, in inner
    raise new_exc from exception
ValueError

We should give the user human-readable error messages here precisely identifying the yaml syntax problem, not a page of scrolly backtrace.