Check that rabbit application is running when trying to enable plugins.

If the node is started, but rabbit application is not running,
plugins can fail to start when enabled.

Check for rabbit application and return an error.

Part of #rabbitmq/rabbimq-cli#226
[#153184802]

Author: Daniil Fedotov

src/rabbit_plugins.erl

@@ -44,7 +44,13 @@
 
 %%----------------------------------------------------------------------------
 
-ensure(FileJustChanged0) ->
+ensure(FileJustChanged) ->
+    case rabbit:is_running() of
+        true  -> ensure1(FileJustChanged);
+        false -> {error, rabbit_not_running}
+    end.
+
+ensure1(FileJustChanged0) ->
     {ok, OurFile0} = application:get_env(rabbit, enabled_plugins_file),
     FileJustChanged = filename:nativename(FileJustChanged0),
     OurFile = filename:nativename(OurFile0),