Improve check on git-svnrevert, better error message

Improve check on git-svnrevert, better error message

When the commit is not in the tree at all, find-rev returns 0
and prints an empty string. We need to catch that problem too,
when trying to revert.

Adding a list of possible problems, so that you can easily and
quickly correct without having to edit the script again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237516 91177308-0d34-0410-b5e6-96231b3b80d8