QDoc: Adding an invalid path to "excludedirs" ruins the doc build

Adding an invalid entry in the excludedirs variable ruins the doc build.
It should silently ignore the incorrect directory.

Observed in the desktop components where the "../private" is in the excludedirs but it was removed after a code refactor. The qdoc build doesn't finish, without any warning as to why it didn't finish.

...took 1.5 weeks to figure that out.