Handle call errors

[coder-with-a-bushido]

Fixes https://famedly-ip.sentry.io/issues/6207590176/?project=1448986&query=is%3Aunresolved%20null%20check%20operator&referrer=issue-stream&statsPeriod=7d&stream_index=3

[td-famedly]

lg, do you maybe want to add a test for it? I think we could add a flag to MockWebRTCDelegate which tells it to throw an error on createPeerConnection and then we can confirm if the call is terminated as expected.

(feel free to modify the suggestion if you have better ideas!)

[codecov]

Codecov Report

❌ Patch coverage is 36.84211% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.61%. Comparing base (510ba9d) to head (1132244).

Files with missing lines	Patch %	Lines
lib/src/voip/call_session.dart	36.84%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2011      +/-   ##
==========================================
+ Coverage   57.54%   57.61%   +0.06%     
==========================================
  Files         153      153              
  Lines       19485    19489       +4     
==========================================
+ Hits        11213    11228      +15     
+ Misses       8272     8261      -11     

Files with missing lines	Coverage Δ
lib/src/voip/utils/types.dart	16.66% <ø> (+16.66%)	⬆️
lib/src/voip/call_session.dart	54.42% <36.84%> (+1.42%)	⬆️

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