Currently the `getPath` methods returned `NULL` in case when a file with the specified ID does not exist. This however mandates that developers are checking for the `NULL` case and if they do not the door for bugs with all kind of impact is widely opened.

This is especially harmful if used in context with Views where the final result is limited based on the result of `getPath`, if `getPath` returns `NULL` PHP type juggles this to an empty string resulting in all possible kind of bugs.

While one could argue that this is a misusage of the API the fact is that it is very often misused and an exception will trigger an immediate stop of execution as well as log this behaviour and show a pretty error page.

I also adjusted some usages where I believe that we need to catch these errors, in most cases this is though simply an error that should hard-fail.

This reverts commit 6a1a4880.

* test case added to avoid adding them later

I guess that should be `default` and not `defautl`, wondering how this ever worked.

After each test suite, detects whether there are stray datafiles, hooks
or proxies, then show a warning and clear them.

Conflicts:
	lib/private/vcategories.php