Handle unavailable registry more gracefully

[FrederikBolding]

This PR makes a few changes to the registry controller implementation to attempt to handle an unavailable registry (e.g. invalid signature) more gracefully.

Instead of throwing when the registry fails to update or populate, we return SnapsRegistryStatus.Unavailable. This status is meant to indicate to the SnapController that we can't currently access the latest registry and thus our data might be out of date. For Snaps that don't require allowlisting, this shouldn't be a problem and they will still be installable. If a user is trying to install a Snap that does require allowlisting they are now greeted with a slightly better error message.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 96.59%. Comparing base (8b461ef) to head (3724684).

Files	Patch %	Lines
...kages/snaps-controllers/src/snaps/registry/json.ts	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2256      +/-   ##
==========================================
- Coverage   96.61%   96.59%   -0.02%     
==========================================
  Files         337      337              
  Lines        7611     7616       +5     
  Branches     1180     1182       +2     
==========================================
+ Hits         7353     7357       +4     
- Misses        258      259       +1     

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