CI: Fail jobs on RuboCop warnings

Usage:
  fail_on_warnings "some command" || exit 1
  run_timed_command "fail_on_warnings some command"

Example:
  $ run_timed_command "fail_on_warnings ls invalid"
  $ fail_on_warnings ls invalid
  There were warnings:
  ls: cannot access 'invalid': No such file or directory
  ==> 'fail_on_warnings ls invalid' failed (1) in 0 seconds.