piston hijacks any TypeError raised by MAAS

[Raphaël Badin]

When a TypeError is raised inside MAAS (indicative of a serious bug), piston hijacks it and returns a response with a 400 status code and a content that looks like this:
"""
Piston/0.2.3rc1 (Django 1.5.4) crash report:

Method signature does not match.

Resource does not expect any parameters.
"""

This is really suboptimal as it obscures the root cause of the error. Even with PISTON_DISPLAY_ERRORS set you don't get the full stacktrace.

[Raphaël Badin (rvb)]

The culprit piston's error_handler() method in resource.py (/usr/share/pyshared/piston/resource.py in the package). The docstring says it can be overridden. We should definitely do that and code up something more sensible (maybe replacing it with an empty method would be all that is needed).