Prerequisites may link to non-existent courses

Example: [PHYS 372](http://qcumber.ca/courses/PHYS_372/) lists and links [PHYS 274](http://qcumber.ca/search/?q=PHYS+274) as an exclusion. That course doesn't exist on solus at the moment, so it just lands on a resultless search, which is unexpected for an internal link.

Brought up by my housemate, @LizabetSullivan.