clang-format: tighten up user facing error messages

If fixes failed to apply, the user most likely needs to do something
manual to their tree to recover.  Add a short error message rather
than throw a traceback.

Bug: 140943979
Test: formatting a tree w/errors showed a nicer message & no traceback
Change-Id: I591fe3895ea648f757b31ba284c910f727eba0da