RepositoryForm can raise many different types of errors. Some are

standard ValidationErrors that are handled by the form. Some are

attributes that get set that the template can render state from. None of

these are really easily distinguished except by the contents of the

error message, making them a bit useless outside of the form's internals

and the front-end page.
This change works toward improving RepositoryForm for usage outside of

the admin page. We now have an error code associated with most types of

ValidationErrors, and we store the error raised during form validation

so we can get to it later (a feature that newer versions of Django have

natively, but we do not). The repository verification code now just

worries about raising the errors, and the calling method stores those

and translates them into the individual attributes.
An upcoming change to the repository API will make use of this.

Unit tests pass.
Tested along with upcoming changes that make use of the new error codes.