Show warning if formatter did not exit normally

I recently made some changes to my .clang-format file that turned out to be incompatible with my version of the executable. In that case, the default invocation of clang-format returns a nonzero value and only outputs a diagnostic message into the error stream. The formatter plugin then returns its input unchanged, with the side effect of autodetecting the indentation width as 1 space if configured to do so.

This was a bit confusing, so I thought it might be helpful to warn the user if this happens. My experience with these tools is limited to clang-format; would it be acceptable to implement it like this for all custom scripts?