Remove `FileNotFound` error

[rylev]

Currently the FileNotFound error does not really represent an error case so much as the user has asked for a file that doesn't exist. This is normally represented as a None in an option. I think we should return ZipResult<Option<ZipFile>> from ZipArchive::by_index and ZipArchive::by_name

[Plecra]

I think the error type could do with some TLC. Is there any reason we shouldn't replace the &'static strs with some more descriptive variants?

[rylev]

I'd like to open a new issue to address that. This should remain just for the FileNotFound case.