DEV: Show message when cannot invite user to PM (#10336)

* DEV: Show message when cannot invite user to PM

When inviting a user to a PM return a message that says, "Sorry, this
user can't be invited." if they have been muted or are not in a users
allowed pm users list.

* Minor refactor & improved some text

app/controllers/topics_controller.rb

@@ -689,7 +689,7 @@ class TopicsController < ApplicationController
 
         render json: json, status: 422
       end
-    rescue Topic::UserExists => e
+    rescue Topic::UserExists, Topic::NotAllowed => e
       render json: { errors: [e.message] }, status: 422
     end
   end