Improve WikiManager.CreateWiki when it's visited by an user without the right to create a wiki

Steps to reproduce:

1. Login with a normal (non Admin) user
2. Go to WikiManager.CreateWiki
3. Fill in the fields in both steps
4. Click Create

You'll get an error similar to this one:

Wiki "test" creation failed: org.xwiki.security.authorization.AccessDeniedException: Access denied when checking [createwiki] access to [Wiki xwiki] for user [xwiki:XWiki.manuel].

An improvement might be if the user got this error message as soon as he visits WikiManager.CreateWiki. There's no point in letting the user fill in the fields if he doesn't have the right to create a wiki.

Also the error message isn't that user friendly. We could use something like

 
Unable to create the wiki "test". You don't have the right to create wikis.