Fixed: enrol role Imprv: Already signed up

Fix:
- enrol_try_internal_enrol() was missing a role param. without this, if
a user wasn't enrolled onto a course facetoface would do it for them but
with no role and they wouldn't be able to see the rest of the course
contents.
* id 5 == student role id in mdl_role

Imprv:
- Clearer already signed up message, showing info on which session the
user is signed up on (so admin can remove if needs be.)