feat: replace anyhow errors in session layer with proper error variants

[davidsteiner]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 92.92929% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.50%. Comparing base (45422d0) to head (d85499b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/hotfix/src/session.rs	87.02%	17 Missing ⚠️
crates/hotfix/src/session_schedule.rs	93.69%	7 Missing ⚠️
crates/hotfix/src/initiator.rs	96.84%	3 Missing ⚠️
crates/hotfix/src/session/error.rs	98.07%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #302      +/-   ##
==========================================
+ Coverage   90.20%   90.50%   +0.30%     
==========================================
  Files          84       85       +1     
  Lines        8394     8608     +214     
==========================================
+ Hits         7572     7791     +219     
+ Misses        822      817       -5     

Flag	Coverage Δ
core	90.36% <92.92%> (+0.31%)	⬆️

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