Fix explorer crash when proving transaction & enhance mithril-client error

[Alenar]

Content

This PR includes:

• Fix explorer crash when an error is returned when proving transaction (#1784)

• Enhance error transmission from the mithril-client when the queried aggregator return a 4xx or a 5xx error.

Pre-submit checklist

• Branch
  • [x] Tests are provided (if possible)
  • [x] Crates versions are updated (if relevant)
  • [ ] CHANGELOG file is updated (if relevant)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested

Issue(s)

Closes #1784

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Test Results

    4 files  ± 0     52 suites  ±0   8m 47s :stopwatch: -5s 1 141 tests + 6  1 141 :white_check_mark: + 6  0 :zzz: ±0  0 :x: ±0  1 307 runs  +24  1 307 :white_check_mark: +24  0 :zzz: ±0  0 :x: ±0 

Results for commit 09f0cadf. ± Comparison against base commit 4b490845.

:recycle: This comment has been updated with latest results.