dtls: don't report error for closed connection

[jkralik]

No description provided.

[codecov-commenter]

Codecov Report

Merging #266 (f2a4377) into master (9c374b0) will decrease coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #266      +/-   ##
==========================================
- Coverage   63.91%   63.70%   -0.22%     
==========================================
  Files          79       79              
  Lines        5640     5640              
==========================================
- Hits         3605     3593      -12     
- Misses       1677     1687      +10     
- Partials      358      360       +2     

Impacted Files	Coverage Δ
dtls/client.go	75.22% <100.00%> (ø)
dtls/server.go	82.78% <100.00%> (ø)
pkg/cache/cache.go	64.00% <0.00%> (-10.00%)	⬇️
net/conn.go	83.60% <0.00%> (-1.64%)	⬇️
net/blockwise/blockwise.go	69.00% <0.00%> (-1.21%)	⬇️

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
42.1% Duplication