Don't cache invalid completer check results

If the check for available completers isn't run because the server isn't
alive, or the check request erred or times out, don't cache the result. Only
cache valid returns.

python/ycm/youcompleteme.py

@@ -222,8 +222,14 @@ class YouCompleteMe( object ):
     except KeyError:
       pass
 
-    exists_completer = ( self.IsServerAlive() and
-                         bool( SendCompleterAvailableRequest( filetype ) ) )
+    if not self.IsServerAlive():
+      return False
+
+    exists_completer = SendCompleterAvailableRequest( filetype )
+    if exists_completer is None:
+      return False
+
+    exists_completer = bool ( exists_completer )
     self._available_completers[ filetype ] = exists_completer
     return exists_completer