The API-sig has a guideline[1] for including error codes in error
responses to help distinguish errors with the same status code
from one another. This change provides a simplest-thing-that-could-
possibly-work solution to make that go.
This solution comes about after a few different constraints and attempts:
* We would prefer to go on using the existing webob.exc exceptions, not
make subclasses.
* We already have a special wrapper around our wsgi apps to deal with
setting the json_error_formatter.
* Though webob allows custom Request and Response objects, it uses the
default Response object as the parent of the HTTP exceptions.
* The Response object accepts kwargs, but only if they can be associated
with known attributes on the class. Since we can't subclass...
* The json_error_formatter method is not passed the raw exception, but
it does get the current WSGI environ
* The webob.exc classes take a 'comment' kwarg that is not used, but
is also not passed to the json_error_formatter.
Therefore, when we raise an exception, we can set 'comment' to a code
and then assign that comment to a well known field in the environ and if
that environ is set in json_error_formatter, we can set 'code' in the
output.
This is done in a new microversion, 1.23. Every response gets a default
code 'placement.undefined_code' from 1.23 on. Future development will
add specific codes where required. This change adds a stub code for
inventory in use when doing a PUT to .../inventories but the name
may need improvement.
[1] http://specs.openstack.org/openstack/api-wg/guidelines/errors.html
Implements blueprint placement-api-error-handling
Change-Id: I9a833aa35d474caa35e640bbad6c436a3b16ac5e
bd9f24b7aa