Gives user notification when a username is already registered.

The server actually sent an event called 'register_error', but there were
nothing client-side that listened for it. This commit mitigates this.
It also clears up any old messages, so that they don't stack up, but rather
flashes when entering multiple bad passwords or already registered usernames.
Merge pull request #297 from hermansc/register-form-alerts

Gives user notification when a username is already registered.