Fix FunctionClauseError when attempting to handle message in error state

[sgfn]

ref: elixir-webrtc/ex_ice#91

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.16%. Comparing base (9af4ebe) to head (6c87349).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   71.95%   72.16%   +0.20%     
==========================================
  Files           8        8              
  Lines         271      273       +2     
==========================================
+ Hits          195      197       +2     
  Misses         76       76              

Files with missing lines	Coverage Δ
lib/ex_turn/client.ex	71.42% <100.00%> (+0.24%)	⬆️

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[Karolk99]

We can add a test for this case, to be sure it won't happen again.