[MRG] minor improvements to Index tests

[ctb]

This PR:

• properly handles JSON parsing errors in MultiIndex.load_from_directory;
• adds a simpler test of that bad JSON raise;
• updates comments, simplifies tests.

[codecov]

Codecov Report

Merging #1900 (68c7edb) into latest (c26ac4e) will increase coverage by 7.90%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1900      +/-   ##
==========================================
+ Coverage   82.82%   90.73%   +7.90%     
==========================================
  Files         122       92      -30     
  Lines       13254     9052    -4202     
  Branches     1789     1789              
==========================================
- Hits        10978     8213    -2765     
+ Misses       2013      576    -1437     
  Partials      263      263              

Flag	Coverage Δ
python	90.73% <100.00%> (ø)
rust	?

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Impacted Files	Coverage Δ
src/sourmash/index/__init__.py	96.73% <100.00%> (ø)
src/core/src/ffi/nodegraph.rs
src/core/src/index/mod.rs
src/core/src/ffi/signature.rs
src/core/src/ffi/mod.rs
src/core/src/encodings.rs
src/core/tests/minhash.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/index/search.rs
src/core/src/ffi/utils.rs
... and 21 more

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[bluegenes]

lgtm