Drop github.com/pkg/errors and fix error_code detection

[na--]

In the spirit of spring cleaning and #1933 😅

This also makes some of the changes that #1660 did, though in a slightly different way, so we should probably try to port the rest of them.

[mstoykov]

LGTM, but I would prefer to get the tests from #1660 ported, before we merge this

[codecov-io]

Codecov Report

Merging #1951 (74bbe0a) into master (3515db5) will increase coverage by 0.04%.
The diff coverage is 58.40%.

❗ Current head 74bbe0a differs from pull request most recent head bd83527. Consider uploading reports for the commit bd83527 to get more accurate results

@@            Coverage Diff             @@
##           master    #1951      +/-   ##
==========================================
+ Coverage   71.26%   71.30%   +0.04%     
==========================================
  Files         181      183       +2     
  Lines       14256    14274      +18     
==========================================
+ Hits        10159    10178      +19     
+ Misses       3478     3476       -2     
- Partials      619      620       +1     

Flag	Coverage Δ
ubuntu	71.24% <55.04%> (+0.02%)	⬆️
windows	70.90% <58.03%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/v1/group.go	89.06% <0.00%> (ø)
cloudapi/api.go	23.86% <0.00%> (ø)
cloudapi/errors.go	60.00% <ø> (ø)
cmd/cloud.go	4.83% <ø> (ø)
cmd/login_cloud.go	0.00% <ø> (ø)
cmd/run.go	12.01% <0.00%> (+0.05%)	⬆️
cmd/scale.go	0.00% <ø> (ø)
converter/har/converter.go	61.17% <0.00%> (-0.73%)	⬇️
js/bundle.go	92.15% <0.00%> (ø)
js/modules/k6/data/data.go	66.66% <ø> (ø)
... and 31 more

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[mstoykov]

LGTM

But the test added in 9fac9ca breaks until d14d24d

[na--]

Yes, I know, I deliberately didn't fix it as a part of the cherry-pick. I'll probably squash this PR when merging it in master, but I'll leave the commits as they are here in the PR, so the iterative issue fixing and refactoring process is clear to anyone that wants to debug it in the future, but we won't have any issues with git bisect or something like that.

[imiric]

LGTM, I like the more descriptive error messages. Just left a couple of tiny nitpicks.