Fix and test ArgumentError for nonexistant dir

[bencottier]

Following #34, The ArgumentError call was not valid, and there was no test for the case where the ArgumentError happens.

[codecov]

Codecov Report

Merging #36 (21cde53) into main (9e0b661) will decrease coverage by 0.51%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
- Coverage   95.51%   95.00%   -0.52%     
==========================================
  Files           5        5              
  Lines         156      140      -16     
==========================================
- Hits          149      133      -16     
  Misses          7        7              

Impacted Files	Coverage Δ
src/indexing.jl	91.89% <100.00%> (-0.42%)	⬇️
src/Checkpoints.jl	91.66% <0.00%> (-1.36%)	⬇️
src/handler.jl	96.77% <0.00%> (-0.20%)	⬇️
src/session.jl	100.00% <0.00%> (ø)

---

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 9e0b661...21cde53. Read the comment docs.