coala: Exit gracefully on SystemExit

If a subroutine raises SystemExit it took care of informing the user
itself. Also we don't want to exit with 1 in case any results were
yielded or so.

coala

@@ -50,7 +50,7 @@ if __name__ == "__main__":
         print(_("Program terminated by user."))
         exit(255)
     except SystemExit:
-        pass
+        exit(0)
     except:
         exception = sys.exc_info()[1]
         UNKNOWN_ERROR = _("An unknown error occurred.") + " " + \