don't send write error in CONNECTION_CLOSE frames

[marten-seemann]

If the send queue already failed, there's no point in trying to send out a CONNECTION_CLOSE.

[codecov]

Codecov Report

Merging #2666 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2666      +/-   ##
==========================================
+ Coverage   86.44%   86.50%   +0.06%     
==========================================
  Files         122      122              
  Lines        9763     9763              
==========================================
+ Hits         8439     8445       +6     
+ Misses        989      986       -3     
+ Partials      335      332       -3     

Impacted Files	Coverage Δ
session.go	77.07% <100.00%> (+0.40%)	⬆️
send_queue.go	100.00% <0.00%> (+7.69%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 953f347...61d8e11. Read the comment docs.