Add missing documentation for API errors.

We introduced a number of API errors since we last updated the
documentation for this, but we never added the API docs. These could
still use some work, but at least we now have something to link to.

Reviewed at https://reviews.reviewboard.org/r/5777/
Revert the change to the recaptcha import test.

For some mysterious reason, recaptcha doesn't always work with
imp.find_module(). It works fine in my virtualenv, but not in Christian's,
despite everything looking very similar.

This changes our test back to using 'import', and crucially adds a comment
explaining why we use that over imp.find_module().

Reviewed at https://reviews.reviewboard.org/r/5776/