Make sure a file exists before searching the parse tree

Since everything is concurrent we can easily find ourselves in a
situation where the occurrence collector is used on a file that
doesn't exist anymore, so now we make sure that files exist
before trying to traverse the parse-tree.

Fix for #57 on Github
Merge pull request #59 from mads379/github-57-check-that-file-exists

Make sure a file exists before searching the parse tree