Self enrolment simply tells "You can not enrol yourself in this course" instead of using the appropriate error strings

Steps to reproduce:
1. Create a course C
2. Disable guest access, enable self enrolment
3. Configure Self enrolment with "max enrolled users" = 1
4. Login as student A
5. Enrol into course C, enrolment should suceedd
6. Login as student B
7. Enrol into course C, enrolment shouldn't be possible.

Actual result:
Student B is shown the error message "You can not enrol yourself in this course" (string 'notenrollable | core_enrol')

Expected result:
Student B should be shown the error message "Maximum number of users allowed to access was already reached" (string 'maxenrolledreached | enrol_self')

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As this has already worked in previous Moodle versions, I think this is a regression.

Hints for finding the bug:
In /enrol/self/lib.php, can_self_enrol() returns the appropriate strings, but enrol_page_hook which calls can_self_enrol() doesn't process these strings and doesn't hand them over to /enrol/index.php. So /enrol/index.php simply displays the "notenrollable" string.