produce more detailed error message when failing to load manifest from zipfile

[ctb]

in collection.rs, we have:

let manifest = Manifest::from_reader(storage.load("SOURMASH-MANIFEST.csv")?.as_slice())?;

which results in an error that looks like this,

Error: Path not found: SOURMASH-MANIFEST.csv

when a zipfile doesn't include a manifest.

I think we should produce a more specific error message to help guide the user.

related issues:

• provide a better error when trying to read a zip file without a manifest sourmash_plugin_branchwater#264