Add admin settings to control error detail exposure in the frontend

When an error occurs XWiki displays a box with the full stack trace and in some cases the actual source code directly to the user. This can be a security issue for some instances.

1. Possible improvements:
   Add a new setting in the admin panel that lets an administrator choose the error verbosity rendered in the wiki. The setting should have at least 2 modes:

• Detail mode: full stack trace (current behavior)
• Generic mode: in this mode XWiki should only display a generic error and an error ID in the frontend and the actual error should be displayed in the the server logs along the id so it is easier to identify