Auto merge of #1898 - mispencer:DontCacheAvailableCheckErrors, r=micbou

Don't cache invalid completer check results

[Lets try this again](https://github.com/Valloric/YouCompleteMe/pull/1897#issuecomment-170978003)

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.

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python/ycm/youcompleteme.py

@@ -222,8 +222,13 @@ 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
+
     self._available_completers[ filetype ] = exists_completer
     return exists_completer