Filter clang-tidy output on Drone for errors

Filter clang-tidy output on Drone for errors

The run-clang-tidy is very verbose and prints a lot of unneccessary
information.  This commit greps for errors in the output.
Moreover, we add the -quiet switch.

As a desired "side" effect, this commit repairs the issue of
run-clang-tidy-6.0 that it does not return a usable exit code.
Hence clang-tidy errors will lead to CI failures now.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>