[MRG] improve lca index error message when inserting num signature

[ctb]

LCA databases cannot be created with num signatures; while lca index does error out, it does so with an uninformative error about downsampling. This improves the lca index error message.

Fixes #1029
Fixes #462

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1076 into master will increase coverage by 9.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1076      +/-   ##
==========================================
+ Coverage   83.63%   92.82%   +9.18%     
==========================================
  Files          99       74      -25     
  Lines        8996     5700    -3296     
==========================================
- Hits         7524     5291    -2233     
+ Misses       1472      409    -1063     

Flag	Coverage Δ
#rusttests	?

Impacted Files	Coverage Δ
sourmash/lca/command_index.py	93.22% <100.00%> (+0.71%)	⬆️
sourmash/lca/lca_db.py	96.29% <100.00%> (-0.90%)	⬇️
sourmash/sbt_storage.py	92.11% <0.00%> (-0.46%)	⬇️
sourmash/sig/__main__.py	92.94% <0.00%> (-0.44%)	⬇️
sourmash/sourmash_args.py	95.19% <0.00%> (-0.18%)	⬇️
src/core/src/wasm.rs
src/core/src/errors.rs
src/core/src/ffi/minhash.rs
src/core/src/sketch/ukhs.rs
... and 26 more

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[ctb]

Ready for review & merge

[bluegenes]

lgtm

[ctb]

thanks!