Files dependencies fixes

I'm working on a different dependency loading mechanism for a current project and I came across two quirks in the `Motion::Project::Config#files_dependencies` method.

The first is that, if a pathname had "./" prepended to it, but then could not be found in the files list, the pathname without "./" was given in the exception instead of the updated filename. More helpful for debugging than anything.

The second issue was that the regex for determining if a pathname is relative did not support the "../" directory ascending notation.

It also looks like there's a bit of trailing whitespace cleanup in the patch.

Thanks for the awesome work!