fix(testing/unstable): report descriptive error when inline snapshot update fails

[minato32]

Closes #6746.

When assertInlineSnapshot() is run with --update and the post-update deno fmt subprocess fails (e.g. the test was run without --allow-run), the original error was thrown from inside the unload handler and surfaced through the test runner as error: null, giving the caller no clue what went wrong.

The unload handler now catches errors from updateSnapshots() and re-throws them wrapped in an Error whose message is prefixed with assertInlineSnapshot: failed to update snapshots: and whose cause is the original error, so both the runner output and any tooling reading cause get the real reason.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.55%. Comparing base (cdf74a8) to head (3b81700).

Files with missing lines	Patch %	Lines
testing/unstable_snapshot.ts	25.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7191      +/-   ##
==========================================
- Coverage   94.57%   94.55%   -0.02%     
==========================================
  Files         636      637       +1     
  Lines       52142    52157      +15     
  Branches     9401     9402       +1     
==========================================
+ Hits        49315    49319       +4     
- Misses       2249     2259      +10     
- Partials      578      579       +1     

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