Print clang-format output

[kjin]

In #60:

A fix for this should be landed shortly (#62); all it does is add an error message if linting succeeds and formatting fails. I'm planning to add parsing clang-format XML output as a follow-up (PRs from your end would be more than welcome too). That being said, it's best to run gts fix to fix format issues anyway; I believe clang-format output is not necessarily meant to be human-interpretable.

When clang-format fails, all we do is print that there were formatting issues. gts fix fixes those issues, but it would still be nice to show the lines of code that need to be addressed.