Error message when admin adds user to institution and quota is full

Bug #1204699 sends a notification to site and institution admins when a user tries to register for an institution whose quota has been reached.

However, the system (Mahara 1.9) still gives an error message only in the error log when an admin tries to add a user to an institution with a full quota. When you don't have error logging to the screen turned on, you only get "Site not available". There should be a proper error message.

Currently, I get:

[WAR] be (lib/institution.php:285) Trying to add a user to an institution that already has a full quota of members
Call stack (most recent first):

    Institution->addUserAsMember(object(User)) at /home/kristina/code/19stable/htdocs/auth/user.php:705
    User->join_institution("careerservice") at /home/kristina/code/19stable/htdocs/admin/users/edit.php:793
    edituser_institution_submit(object(Pieform), array(size 8)) at Unknown:0
    call_user_func_array("edituser_institution_submit", array(size 2)) at /home/kristina/code/19stable/htdocs/lib/pieforms/pieform.php:527
    Pieform->__construct(array(size 5)) at /home/kristina/code/19stable/htdocs/lib/pieforms/pieform.php:162
    Pieform::process(array(size 5)) at /home/kristina/code/19stable/htdocs/lib/pieforms/pieform.php:71
    pieform(array(size 5)) at /home/kristina/code/19stable/htdocs/admin/users/edit.php:733

Testing was done on Mahara 1.9.0, an upgraded Mahara 1.8 site