Convert Dashboard's ArtifactNotFound exception to Optuna's one

[c-bata]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

Follow-up #540 and #752.

What does this implement/fix? Explain your changes.

Convert Dashboard's ArtifactNotFound exception to Optuna's one

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (99a54cb) 68.81% compared to head (fd4ef3d) 69.48%.
Report is 20 commits behind head on main.

Files	Patch %	Lines
optuna_dashboard/artifact/_backend_to_store.py	54.54%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #753      +/-   ##
==========================================
+ Coverage   68.81%   69.48%   +0.67%     
==========================================
  Files          35       35              
  Lines        2344     2353       +9     
==========================================
+ Hits         1613     1635      +22     
+ Misses        731      718      -13     

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