[RFC SAMPLE] builtin/branch: give more useful error messages when renaming

The patch series results in a change in output as specified below. Only few cases have been shown here to keep it short. The output for other cases are similar.

$ git branch
* master
   foo
   bar

Before patch,

$ # Trying to rename non-existent branch $ git branch -m hypothet no_such_branch error: refname refs/heads/hypothet not found fatal: Branch rename failed

$ # Trying to rename non-existent branch into existing one $ git branch -m hypothet master error: refname refs/heads/hypothet not found fatal: Branch rename failed

$ # Trying to force update current branch $ git branch -M foo master fatal: Cannot force update the current branch.

$ # Trying to force rename an in-existent branch with an invalid name $ git branch -M hypothet ?123 fatal: '?123' is not a valid branch name.

After patch,

$ # Trying to rename non-existent branch $ git branch -m hypothet no_such_branch fatal: branch 'hypothet' doesn't exist

$ # Trying to rename non-existent branch into existing one $ git branch -m hypothet master fatal: branch 'hypothet' doesn't exist, and branch 'master' already exists

$ # Trying to force update current branch $ git branch -M foo master fatal: cannot force update the current branch

$ # Trying to force rename an in-existent branch with an invalid name $ git branch -M hypothet ?123 fatal: branch 'hypothet' doesn't exist, and branch name '?123' is invalid