Display 404 page instead of NPE stacktrace page.

Displays a 404 if no update was found for the given ID instead of
throwing a NullPointerException.

Fixes #145.
Only accept numbers as ID.

Displays a 404 instead of a NumberFormatException.

Relates to #145.
Merge pull request #146 from mvitz/145-npe-for-not-used-id

Fixes #145 by displaying 404 page instead of stacktrace page.