Show plugin name if cannot start it (#2111)

src/emqx_plugins.erl

@@ -170,10 +170,13 @@ list() ->
 
 plugin(CfgFile) ->
     AppName = app_name(CfgFile),
-    {ok, Attrs} = application:get_all_key(AppName),
-    Ver = proplists:get_value(vsn, Attrs, "0"),
-    Descr = proplists:get_value(description, Attrs, ""),
-    #plugin{name = AppName, version = Ver, descr = Descr}.
+    case application:get_all_key(AppName) of
+        {ok, Attrs} ->
+            Ver = proplists:get_value(vsn, Attrs, "0"),
+            Descr = proplists:get_value(description, Attrs, ""),
+            #plugin{name = AppName, version = Ver, descr = Descr};
+        undefined -> error({plugin_not_found, AppName})
+    end.
 
 %% @doc Load a Plugin
 -spec(load(atom()) -> ok | {error, term()}).