Added required file check

Current exception will catch update-this-repo.sh not existing as a ProcessError, and display the error "script not up to date, please re-run updater.py", assuming the only error that this would catch is an update error. If the user moves this file outside of the utils dir for some reason, or copies it into the brewpi-script dir from an external brewpi-tools repo (and not the other required files), this error would be misleading. This patch checks to see if required file exist in same dir and suggest a solution.