curtin output badly broken. command errors ignored.

This regressed with the 'reporter' code in revno 163.
The following shows a number of issues:
 a.) cannot run as non-root (as it wants to write to /var/log/curtin_install.log)
 b.) failure codes are ignored causing stage commands to run even after one fails
 c.) output is duplicated
 d.) *old* output from /var/log/curtin_install.log is written to console

$ cat foo.cfg
stages: ['foo']
foo_commands:
  builtin: ['sh', '-c', 'echo FOO_BUILTIN; exit 1;']
  zz: ['sh', '-c', 'echo FOO_ZZ']

$ cat bar.cfg
stages: ['bar']
bar_commands:
  builtin: ['sh', '-c', 'echo BAR_BUILTIN; exit 1;']
  zz: ['sh', '-c', 'echo BAR_ZZ']

$ sudo rm /var/log/curtin_install.log

$ ./bin/curtin -v install --config=foo.cfg my-source
'reporter' not found in config file.
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/ubuntu/curtin.dist/curtin/commands/install.py", line 121, in _run_thread
    with io.open(INSTALL_LOG, 'a') as writer, \
IOError: [Errno 13] Permission denied: '/var/log/curtin_install.log'

Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/ubuntu/curtin.dist/curtin/commands/install.py", line 121, in _run_thread
    with io.open(INSTALL_LOG, 'a') as writer, \
IOError: [Errno 13] Permission denied: '/var/log/curtin_install.log'

[Errno 13] Permission denied: '/var/log/curtin_install.log'

$ sudo ./bin/curtin -v install --config=foo.cfg my-source
'reporter' not found in config file.

FOO_BUILTIN
FOO_BUILTIN

FOO_BUILTIN
FOO_BUILTIN

FOO_ZZ
FOO_ZZ

$ sudo ./bin/curtin -v install --config=bar.cfg my-source
'reporter' not found in config file.
FOO_BUILTIN
FOO_ZZ
FOO_BUILTIN
FOO_ZZ
Installation finished.
Installation finished.BAR_BUILTIN
BAR_BUILTIN

FOO_BUILTIN
FOO_ZZ
Installation finished.BAR_BUILTIN
FOO_BUILTIN
FOO_ZZ
Installation finished.BAR_BUILTIN

BAR_ZZ
BAR_ZZ