There are two issues with lock exceptions right now:

- first, we don't log the original error; this is fine for now
  (locking.py always returns the same error here), but in general is
  brittle: if locking.py would start returning more information, we'd
  completely miss that

- second, an actual honest lock conflict is not an internal error;
  it's simply an optimistic lock failing, and as such we should not
  return internal error, but rather resource_not_unique

This addresses issue 287.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>