[MRG] catch TypeError in search w/abund vs flat at the command line

[ctb]

This PR adjusts search to catch an exception that shouldn't be exposed to the user; here the exception is caused by searching a flat database with an abund signature.

[codecov]

Codecov Report

Merging #1928 (1797943) into latest (f0726c3) will increase coverage by 8.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1928      +/-   ##
==========================================
+ Coverage   82.94%   90.97%   +8.03%     
==========================================
  Files         125       94      -31     
  Lines       13759     9483    -4276     
  Branches     1877     1877              
==========================================
- Hits        11412     8627    -2785     
+ Misses       2075      584    -1491     
  Partials      272      272              

Flag	Coverage Δ
python	90.97% <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/commands.py	88.55% <100.00%> (+0.05%)	⬆️
src/core/src/index/mod.rs
src/core/src/cmd.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/ffi/storage.rs
src/core/src/index/revindex.rs
src/core/src/index/search.rs
src/core/src/ffi/nodegraph.rs
src/core/src/signature.rs
src/core/src/ffi/minhash.rs
... and 22 more

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[ctb]

@sourmash-bio/devs ready for review & merge!

[ctb]

still ready for review & merge @sourmash-bio/devs :)

[bluegenes]

lgtm