crypto: Make the return type of is_owner_of_session more specific

[andybalaam]

Make the return type of is_owner_of_session more specific, so we can use it in contexts that expect e.g. an OlmError instead of MegolmError.

Part of #3751

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.08%. Comparing base (d7224a7) to head (81cc3b9).

Files	Patch %	Lines
crates/matrix-sdk-crypto/src/error.rs	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3774      +/-   ##
==========================================
- Coverage   84.09%   84.08%   -0.01%     
==========================================
  Files         259      259              
  Lines       27089    27094       +5     
==========================================
+ Hits        22780    22782       +2     
- Misses       4309     4312       +3     

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