Provide more specific feedback for username errors during registration

- Distinguish between usernames that contain invalid characters and those that
  are blacklisted
- Provide a more accurate list of characters allowed in usernames

Fixes #4939

Props hnla

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7163 cdf35c40-ae34-48e0-9cc9-0c9da1808c22