use the new error types to log the reason why a connection is closed

[marten-seemann]

Reaping the benefits of the massive refactor: more than 200 LOC removed.

@mvdan, can I ask you for a review again?

[codecov]

Codecov Report

Merging #3166 (42ecbf0) into master (5f2cf04) will decrease coverage by 0.03%.
The diff coverage is 96.43%.

@@            Coverage Diff             @@
##           master    #3166      +/-   ##
==========================================
- Coverage   85.38%   85.36%   -0.03%     
==========================================
  Files         132      131       -1     
  Lines        9831     9800      -31     
==========================================
- Hits         8394     8365      -29     
+ Misses       1063     1061       -2     
  Partials      374      374              

Impacted Files	Coverage Δ
qlog/types.go	84.54% <ø> (+0.46%)	⬆️
qlog/event.go	98.11% <96.00%> (+0.11%)	⬆️
logging/multiplex.go	95.24% <100.00%> (ø)
qlog/qlog.go	91.07% <100.00%> (ø)
session.go	77.46% <100.00%> (-0.37%)	⬇️

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