Add acceptance test with non-existing files

Acceptance test with plugin adding entries to `files` object that don't
correspond to real files.
Add unit test for non-existing file error
Implement skipping of non-existing files

If any other plugin leaves `files` with entry that does not correspond
to an existing file this entry should be skipped silently.
Merge pull request #1 from jkuczm/non-existing-files

Non existing files