[MRG] fix sig check --fail-if-missing

[ctb]

Fix sig check from #1907 so that if --fail-if-missing is specified and nothing is missing, it succeeds 😆 .

[codecov]

Codecov Report

Merging #1917 (cab1fba) into latest (51ba09e) will increase coverage by 7.86%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1917      +/-   ##
==========================================
+ Coverage   82.96%   90.83%   +7.86%     
==========================================
  Files         123       93      -30     
  Lines       13367     9165    -4202     
  Branches     1808     1808              
==========================================
- Hits        11090     8325    -2765     
+ Misses       2013      576    -1437     
  Partials      264      264              

Flag	Coverage Δ
python	90.83% <100.00%> (ø)
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/sig/__main__.py	93.56% <100.00%> (ø)
src/core/tests/minhash.rs
src/core/src/encodings.rs
src/core/src/ffi/signature.rs
src/core/src/sketch/nodegraph.rs
src/core/src/lib.rs
src/core/src/index/sbt/mod.rs
src/core/src/ffi/minhash.rs
src/core/src/index/search.rs
src/core/src/storage.rs
... and 21 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 51ba09e...cab1fba. Read the comment docs.

[ctb]

ready for review @sourmash-bio/devs 🐑 😁

[bluegenes]

lgtm!