Properly catch TitoException

This is for example raised by builders, e.g.

    ERROR: Builder missing required argument: mock

src/tito/cli.py

@@ -908,6 +908,9 @@ def main():
     """Command line's entry point."""
     try:
         CLI().main(sys.argv[1:])
+    except TitoException:
+        e = sys.exc_info()[1]
+        error_out(e.message)
     except KeyboardInterrupt:
         pass