Currently when `repository#add_branch` returns `false` in
`Branches::CreateService` we return error message that the source
reference was invalid. This is the most likely cause of this error, but
not the only one.

Another cause is creating a branch with a subcomponent that matches the
full name of an existing branch, e.g. `feature/widget` and `feature`. In
this scenario it is the new branch name that is the problem, not the
source ref, but our error message gives no clues to this.

This is a particular problem for pull mirroring, where the SHA1 of the
source branch is used rather than a human-readable name. This requires
the user to copy down the SHA and go to the source repo to attempt to
understand which branch is causing the failure.

Ideally we would have separate errors for invalid source refs and
invalid branch names, but `repository#add_branch` hides the information
we need to distinguish between them.

This commit expands the error message to mention which branch we failed
to create, making it easier for users to find out they were creating
invalid branch names.