Trying to display non-existing printer/class in web ui doesn't return 404.

[zdohnal]

I took this one from https://github.com/apple/cups/issues/6045 . Currently we see an empty template if you go to web interface page for non-existing printer/class f.e. (class testclass and printer testprinter don't exist):

http://localhost:631/classes/testclass
http://localhost:631/printers/testpriner

We should show error when we try to access an non-existing printer directly by uri.