Include link to login page into 'noguestaccess' message on enrolment screen

noguestaccess message (in enrol.php) should include a link to login page, to allow guest users (non already connected users) to connect easilly.

Because most of the time, users don't spend time to really read the "error message", just think there's a problem, and don't look at the page to see where/how to connect. Hence the idea to directly put the link IN the message.

The message "Guests can not access this course, please try to log in." would be better with a link.