improve log-level and exit-code when rexfile is in use

[scorpevans]

Reference: https://github.com/adjust/Rex/blob/master/lib/Rex/CLI.pm#L638

When trex is invoked in parallel without the -F flag, it silently logs an INFO (Rexfile is in use) and exits with zero.
This behaviour is dangerous for client which use the error code to determine if everything went well; this is not so in such a case when one entire processing thread has been skipped.

I propose a WARNING along with a non-zero exit code.