The process of checking repositories for conflicts (by name and by path)

required two SQL queries, one for each field. While not terribly

inefficient, it could certainly be improved. As could the error

information.
This shortens the verification process down to one SQL query, which

checks for both name and path conflicts at once. If both have conflicts,

then they'll both be represented in the ValidationError, instead of

only the first one that's found. That exception now comes with a

repository_exists error code, for later matching.
Constants have also been added for the error messages used in the

exception. This is here for an upcoming change to RepositoryForm that

checks if there's a repository conflict. Unfortunately, in Django 1.6,

the ValidationError codes are discarded by the time this logic would

need to be performed, so we're matching strings instead. In the future,

this could be updated to use the codes and drop the constants.

Unit tests pass.
Used this along with some new logic in upcoming tests. Verified it

works as expected.