chore(backend): better error message for unsupported attestation

[saschanaz]

What

#8691 に最低限の対応

Why

せめてなにがunsupportedなのか分かるように

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment (N/A)
• (If needed) Add story of storybook (N/A)
• (If needed) Update CHANGELOG.md (N/A)
• (If possible) Add tests (N/A)

[codecov]

Codecov Report

Merging #11333 (9d0e7d8) into develop (509e3f9) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop   #11333      +/-   ##
===========================================
+ Coverage    77.68%   77.72%   +0.03%     
===========================================
  Files          917      919       +2     
  Lines        92563    92722     +159     
  Branches      7748     7748              
===========================================
+ Hits         71908    72065     +157     
- Misses       20655    20657       +2     

Impacted Files	Coverage Δ
...backend/src/server/api/endpoints/i/2fa/key-done.ts	87.87% <0.00%> (ø)

... and 3 files with indirect coverage changes

[syuilo]

👍