Drop error dialog, if tags are being build

Fixes #223

The error is thrown whenever a command is checked for being enabled.
That's useless as related menu items etc. are disabled anyway and a message in
status bar is already being printed.

plugins/cmds.py

@@ -558,8 +558,6 @@ def check_if_building(self, **args):
     """
     if RebuildTags.build_ctags.func.running:
         status_message("Tags not available until built")
-        if setting("display_rebuilding_message"):
-            error_message("Please wait while tags are built")
         return False
     return True