Fixes Issue #423 :
When user open the api for any unknown classes or printers, then previously it was showing a random UI with that printer/class name.
Now it will throw "ERROR : No class/printer found " error for those cases.
Example for : http://localhost:631/classes/testclass or http://localhost:631/printers/testpriner
@tillkamppeter please review the PR
Fixes Issue #423 : When user open the api for any unknown classes or printers, then previously it was showing a random UI with that printer/class name. Now it will throw "ERROR : No class/printer found " error for those cases. Example for : http://localhost:631/classes/testclass or http://localhost:631/printers/testpriner @tillkamppeter please review the PR