[MRG] check to make sure that .zip files exist before trying to load from them.

[ctb]

This PR changes ZipStorage.load(...) to check to see if a zipfile exists before trying to open it. In turn, this means that an empty zipfile will no longer be created by ZipStorage :)

Fixes #1774

• needs a new test for non-existent zip file!

[codecov]

Codecov Report

Merging #1777 (563dd55) into latest (91ed12f) will increase coverage by 6.69%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1777      +/-   ##
==========================================
+ Coverage   83.49%   90.19%   +6.69%     
==========================================
  Files         113       87      -26     
  Lines       12159     8463    -3696     
  Branches     1621     1622       +1     
==========================================
- Hits        10152     7633    -2519     
+ Misses       1749      572    -1177     
  Partials      258      258              

Flag	Coverage Δ
python	90.19% <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/index.py	95.96% <100.00%> (+0.01%)	⬆️
src/sourmash/sourmash_args.py	92.74% <100.00%> (+0.04%)	⬆️
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/from.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/index/sbt/mod.rs
src/core/src/index/search.rs
src/core/src/index/bigsi.rs
src/core/src/ffi/mod.rs
src/core/src/ffi/minhash.rs
... and 18 more

---

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[ctb]

Ready for review (and, assuming tests pass, merge :) -- @sourmash-bio/devs

[mr-eyes]

LGTM!

[mr-eyes]

I reran the Python tests.

[ctb]

k - looks like the tests are failing for legitimate reasons now :). so I gotta fix that.