The handing by "git branch --set-upstream-to" against various forms
of errorneous inputs were suboptimal.
* jk/set-upstream-error-cases:
branch: give advice when tracking start-point is missing
branch: mention start_name in set-upstream error messages
branch: improve error message for missing --set-upstream-to ref
branch: factor out "upstream is not a branch" error messages
t3200: test --set-upstream-to with bogus refs