Merge pull request #46 from hannobraun/bug/abstract-erase

(fix) Error::abstract was renamed to erase
(fix) Return 404 response, if file was not found

I'm not too happy about this implementation, but I don't really know how
to improve it. I've added comments explaining my misgivings.
Improve error handling code

This fixes one of the warts from the last commit that I was unhappy
about.
(cleanup) Simplify the code that return 404 response