UserCreateBranch: Improve "reference is ambiguous" errors attempt 2

This reverts merge request !4962 (merged) - Fixes for the previous failure are in gitlab!101995 (merged)

#4031 (closed)

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).