push: fold unpack_ok() into finish()

The push cannot be successful if we sent a bad packfile. We should
return an error in that case instead of storing it elsewhere.
push: reword comment on finish()

This should make it clearer what the return value implies.
Merge pull request #2756 from libgit2/cmn/push-error-concerns

Fold `git_push_unpack_ok()` into `git_push_finish()`