Clarify error message for a user attempting to register with capital
letters on a multisite install.

Because WP multisite does not allow a user to register with capital
letters, we need to do an explicit check for capital letters and throw
up a better error message to avoid confusion.

Fixes #5175.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7382 cdf35c40-ae34-48e0-9cc9-0c9da1808c22