[MRG] fix, refactor, and test sourmash sketch output information.

[ctb]

This PR:

• fixes a misplaced ' after the 'CC0' text, a nit introduced by mistake in [MRG] fix bug where sourmash sketch ... --singleton -o output.sig drops signatures #1810
• refactors the panic handling code caused by Rust/Python mismatch (ref fix rust Signature API for minhashes method. #1167) and moves it over to sourmash_args, where it is arguably better handled.
• in the process of refactoring the panic handling code, fixes a problem with zipfile output where signatures were getting double-counted due to the panic handling code.

This PR also adds explicit checks on sourmash output signature counts, which hopefully will start locking down the repeated mistakes I'm making when I change this code 🙄 .

Fixes #1824

[codecov]

Codecov Report

Merging #1826 (1a0d5ba) into latest (d361539) will increase coverage by 6.71%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1826      +/-   ##
==========================================
+ Coverage   83.48%   90.20%   +6.71%     
==========================================
  Files         113       87      -26     
  Lines       12183     8481    -3702     
  Branches     1627     1628       +1     
==========================================
- Hits        10171     7650    -2521     
+ Misses       1754      573    -1181     
  Partials      258      258              

Flag	Coverage Δ
python	90.20% <100.00%> (+0.04%)	⬆️
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/command_compute.py	93.35% <100.00%> (+1.15%)	⬆️
src/sourmash/sourmash_args.py	92.88% <100.00%> (+0.13%)	⬆️
src/core/src/ffi/utils.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/ffi/mod.rs
src/core/src/sketch/minhash.rs
src/core/tests/test.rs
src/core/src/index/bigsi.rs
src/core/src/errors.rs
src/core/src/index/sbt/mhbt.rs
... and 18 more

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[ctb]

Ready for review, and probably merge, @sourmash-bio/devs.

[mr-eyes]

Looks good!

[ctb]

thanks!