missing program, agency or category dumps stack should 404 page not found

[Dan MacNeil]

If a user (usually a spider) accesses an agency,program or category id
wthat doesn't exit, the application dumps stack sends at admin email.

ie:
[...]
Assertion failed at line 321 of Guide.pm : Got no programs for category
803223; expected at least one.
[...]

We should write a method in the base class to return the http headers
for 404 "page not found"

This will reduce clutter in admin email boxes and give spiders and
crackers the feedback they need to stop checking pages that don't exist.

This bug might relate to a more general routine to handle bad URLs
for example Malware trying SQL injection