Throw IllegalArgumentException in CrowdCredentialProvider's changePassword to throw a friendlier 500 page exception (JRA-13685)

At the moment, if an internal directory has a regex for password specified, JIRA will throw an ugly 500 page with an ImmutableException (from the User class). Spoke with Jed about not catching any errors and, instead, throwing an IllegalArgumentException with a friendlier message.