:YcmCompleter<TAB> now won't print traceback

If there's no semantic completer set for the current filetype,
:YcmCompleter<TAB> would print an ugly traceback. Ugh.

Fixes #989.

python/ycm/youcompleteme.py

@@ -196,8 +196,11 @@ class YouCompleteMe( object ):
 
   def GetDefinedSubcommands( self ):
     if self._IsServerAlive():
+      try:
         return BaseRequest.PostDataToHandler( BuildRequestData(),
                                              'defined_subcommands' )
+      except ServerError:
+        return []
     else:
       return []