[MRG] better handle some pickfile errors

[ctb]

This PR adds error handling for situations where the picklist pickfile doesn't exist, or is not a regular file, or is empty, or is not a CSV file.

[codecov]

Codecov Report

Merging #1924 (84594f2) into latest (a4d7e2c) will increase coverage by 7.79%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1924      +/-   ##
==========================================
+ Coverage   83.22%   91.02%   +7.79%     
==========================================
  Files         124       94      -30     
  Lines       13601     9404    -4197     
  Branches     1861     1863       +2     
==========================================
- Hits        11320     8560    -2760     
+ Misses       2014      577    -1437     
  Partials      267      267              

Flag	Coverage Δ
python	91.02% <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/picklist.py	91.39% <100.00%> (+0.29%)	⬆️
src/sourmash/sourmash_args.py	93.40% <100.00%> (ø)
src/core/src/index/revindex.rs
src/core/src/cmd.rs
src/core/src/storage.rs
src/core/src/lib.rs
src/core/src/ffi/index/revindex.rs
src/core/src/from.rs
src/core/src/index/bigsi.rs
src/core/src/ffi/index/mod.rs
... and 22 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 a4d7e2c...84594f2. Read the comment docs.

[bluegenes]

lgtm