Improve permission warning

Improve permission warning

Improve permission warning when Suricata-update runs with the wrong user

When suricata-update runs with a non-root user, it gives an ugly traceback.
To avoid those ugly tracebacks, try except block is put around the operation
that are being performed on the file i.e. shutil.copyfile(src, dest) and
exit cleanly with an error in the log.
Also, to avoid ugly tracebacks for permission of suricata.yaml, try-except
is put around where it is accessed and exit cleanly.