fix(ws): wrongly sending nil as error when write on close fails

[mstoykov]

No description provided.

[mstoykov]

I am expecting that this is the root cause of an issue reported in the community forum, as this is the only place where a nil error can be send :)

I would've added tests but we are using some service to test websockets and I didn't find a way to have a write error with it in the 5 minutes I looked into it ... so maybe adding some priority to #537 and specifically the part with websockets ... it truly shouldn't be that hard to have something like httpbin for it

[codecov-io]

Codecov Report

Merging #1118 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1118      +/-   ##
==========================================
+ Coverage   73.54%   73.59%   +0.05%     
==========================================
  Files         144      144              
  Lines       10531    10530       -1     
==========================================
+ Hits         7745     7750       +5     
+ Misses       2327     2322       -5     
+ Partials      459      458       -1

Impacted Files	Coverage Δ
js/modules/k6/ws/ws.go	81.3% <100%> (+2.35%)	⬆️

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[na--]

We already have websocket tests in the httpmultibin, so it shouldn't be too difficult to add a test for this? https://github.com/loadimpact/k6/blob/f2dff90edc83925399e738195e5617eaf7145398/lib/testutils/httpmultibin.go#L172-L173

Also, please write a PR description suitable for the release notes