[MRG] clean up zip error handling for bad zip files

[ctb]

SaveSignatures_* and ZipStorage have some ugly and slightly broken error handling when trying to open a .zip file that is not, in fact, a valid zipfile. This made it hard to figure out what the actual error was!

This PR cleans up the error handling to present the user with a somewhat more useful error message.

Fixes #2269

[codecov]

Codecov Report

Merging #2270 (924fc24) into latest (3880109) will increase coverage by 7.34%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #2270      +/-   ##
==========================================
+ Coverage   84.84%   92.19%   +7.34%     
==========================================
  Files         131      100      -31     
  Lines       15662    11393    -4269     
  Branches     2248     2250       +2     
==========================================
- Hits        13289    10504    -2785     
+ Misses       2082      598    -1484     
  Partials      291      291              

Flag	Coverage Δ
python	92.19% <100.00%> (+<0.01%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/sbt_storage.py	89.18% <100.00%> (+0.03%)	⬆️
src/sourmash/sourmash_args.py	93.66% <100.00%> (+0.05%)	⬆️
src/core/tests/storage.rs
src/core/src/index/linear.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/index/search.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/ffi/utils.rs
src/core/tests/test.rs
src/core/src/storage.rs
... and 23 more

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[ctb]

ready for review @sourmash-bio/devs @dkoslicki!

[mr-eyes]

LGTM!