IWYU: Emit stderr when expected output is not found

!8406 (merged) introduced the ability to return a non-zero error code from IWYU invocation if the IWYU command line contains -Xiwyu --error. Unfortunately, this also means that if the file being linted has syntax errors, or otherwise fails to compile, IWYU will return a non-zero error code, which will be propagated by CMake. This failure suppresses invocation of the original compile command with the end result of a silent failure from IWYU and no other error messages.

To avoid this we're now checking for a "success" message from IWYU in addition to suggestions for adding/removing headers. If neither the success string nor add/remove suggestions are found, and IWYU exited with an error code, then we'll display stderr returned from the IWYU invocation so that the user can see why the tool failed.