Check that all .graph files listed in one of the GRAPHFILES actually exist

We currently check that all the actual .graph files exist in one of the
GRAPHFILES, it makes sense to check this as well.
Merge pull request #1038 from ronawho/update-checkGraphFiles

Check that all .graph files listed in one of the GRAPHFILES actually exist

[reviewed by @thomasvandoren]

We currently check that all the actual .graph files exist in one of the
GRAPHFILES, it makes sense to check this as well.