[clang-format] Improve clang-format-diff.py error message

Previously if we couldn't run the clang-format command
for some reason, you'd get an unhelpful error message:
```
OSError: [Errno 2] No such file or directory
```

Which doesn't tell you what was happening to cause this.

Catch the error and add the command we were attempting to run:
```
RuntimeError: Failed to run "<...>/clang-food <...>" - No such file or directory"
RuntimeError: Failed to run "<...>/clang-format <...>" - Permission denied"
```

Reviewed By: krasimir

Differential Revision: https://reviews.llvm.org/D98032