fix: Do not emit :registryUpdated on failure

[FrederikBolding]

BugBot indicated that my previous PR had a infinite loop in case of the registry failure state: #3948 (comment)

We do not need to emit :registryUpdated on failure, which should fix this problem.

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Note

Low Risk
Low risk: a small behavioral change to event emission that only affects failure paths, reducing the chance of update-trigger loops when the registry is unavailable.

Overview
Prevents SnapRegistryController from publishing the SnapRegistryController:registryUpdated event when #update() fails (e.g., fetch/verify errors), while still marking databaseUnavailable in state.

Successful updates and no-op updates (recent fetch or unchanged signature) continue to emit :registryUpdated as before.

^{Reviewed by Cursor Bugbot for commit 8af7c86. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.56%. Comparing base (d952951) to head (8af7c86).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3949      +/-   ##
==========================================
- Coverage   98.56%   98.56%   -0.01%     
==========================================
  Files         427      427              
  Lines       12345    12344       -1     
  Branches     1939     1939              
==========================================
- Hits        12168    12167       -1     
  Misses        177      177              

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