API improvements.

In the case of failures in the API use crits_response() now, instead of
raising a BadRequest exception. This extends the work Mike did in this
area.

In cases where we have a URL also return the object ID. This will make
life easier on consumers that only want the object ID. Without this the
only way to get it is to parse it out of the URL.

Fix the Certificate and PCAPs API since it never worked after the
s/parent/related/ change by brlogan.

Fix the Indicator API to require object_id and description as those are
required by the handler to not be None.

Fix an issue in add_objects() where the exception was not cast to string.
This was causing the response back from tastypie to not serialize properly.

Fix logic in screenshots/api.py to properly recognize missing fields.

Fix some minor whitespace problems.