refactor(crypto): Provide a specific error type for to-device events from dehydrated devices

[andybalaam]

This will get more usage soon, when we add a variant for events with unverified senders.

Prep for #4147

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.19%. Comparing base (22c99f3) to head (ec7a04c).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-crypto/src/machine/mod.rs	71.42%	4 Missing ⚠️
...ates/matrix-sdk-crypto/src/machine/test_helpers.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5282      +/-   ##
==========================================
- Coverage   90.19%   90.19%   -0.01%     
==========================================
  Files         334      334              
  Lines      105136   105142       +6     
  Branches   105136   105142       +6     
==========================================
  Hits        94830    94830              
- Misses       6242     6248       +6     
  Partials     4064     4064              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[poljar]

Sensible, the fact that we're not unwedging sessions from dehydrated devices anymore is a win as well.