Clean up error handling in zip creation

We currently condense a whole lot of `std::io::Error`s into one handler, without being able to specify _which_ file exactly threw the error. This is not good, and should be improved.

cc #38

Start by tracing the zip-creation path and locating where multiple std::io::Errors are condensed into one handler. Review the related context in issue #38. Done means errors identify the specific file involved instead of reporting only a generic I/O failure.

Written by the indexing model from the issue text.