UserCreateBranch: Improve "reference is ambiguous" errors
There were a couple of problems here:
- When the "reference is ambiguous" error message trickled up to sentry, it was difficult to understand what was going wrong.
- CreateUserBranch RPC was incorrectly saying it failed, when in actual fact the branch that was being created was not valid (git would refuse too).
Edited by James Fargher