Treat errors from Throw() as plain goja values

[imiric]

This avoids outputting "GoError" as mentioned in #877 (comment) .

I split it off from #1769 to avoid the clutter in that PR.

[na--]

Hmm I'm not sure this is the way to fix the issue. Even if we don't use goja's GoError, we probably should use a custom error wrapper type of our own, so we can then type assert by it.

[imiric]

@na-- I tried porting lib.Exception added in #1769 and panicking with it in Throw(), but it messes up the logged stack trace when k6.fail() is used, and we'd have to resort to manually building the stack trace as done in https://github.com/loadimpact/k6/blob/ccffbcaaddd7f2d472cdced0da847fde2140ef88/js/runner.go#L627-L637

This approach doesn't produce the same trace as what goja produces internally. Before (as done in #1769):

ERRO[0000] fail: failed
        at github.com/loadimpact/k6/js/common.Bind.func1 (native)
        at another (file:///home/ivan/Documents/Load%20Impact/tests/test-877-fail.js:6:13(4))
        at myfail (file:///home/ivan/Documents/Load%20Impact/tests/test-877-fail.js:14:10(2))
        at file:///home/ivan/Documents/Load%20Impact/tests/test-877-fail.js:28:11(6)  executor=shared-iterations scenario=default

After (with the ported lib.Exception):

ERRO[0000] failed
file:///home/ivan/Documents/Load%20Impact/tests/test-877-fail.js:18:1(26)

Where did you consider adding a type assertion for it? I also think an internal error type would make sense here, but not if it messes up the current logging.

[na--]

Sorry for the 6-month late response... 😅 I think we discussed this in Slack, but still should have said something here... 🤦‍♂️

Anyway, prompted by #2049, I just tried your fix (replacing panic(rt.NewGoError(err)) with panic(rt.ToValue(err)) in js.common.Throw() on top of #2046 and it actually seems to work like a charm! Stack traces and exit codes are preserved and GoError disappears, so we should probably release this even in k6 v0.33.0?

[codecov-commenter]

Codecov Report

Merging #1775 (11c4b8a) into master (da5eebc) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 11c4b8a differs from pull request most recent head 47597c9. Consider uploading reports for the commit 47597c9 to get more accurate results

@@            Coverage Diff             @@
##           master    #1775      +/-   ##
==========================================
- Coverage   71.73%   71.70%   -0.03%     
==========================================
  Files         179      179              
  Lines       14100    14100              
==========================================
- Hits        10115    10111       -4     
- Misses       3347     3351       +4     
  Partials      638      638              

Flag	Coverage Δ
ubuntu	71.62% <100.00%> (-0.05%)	⬇️
windows	71.34% <100.00%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
js/common/util.go	68.75% <100.00%> (ø)
loader/loader.go	79.28% <0.00%> (-3.58%)	⬇️
lib/executor/vu_handle.go	95.23% <0.00%> (+0.95%)	⬆️

---

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 da5eebc...47597c9. Read the comment docs.

[na--]

LGTM. We can either merge this first and I will rebase #2046, or the other way around, 🤷‍♂️ will be simpler, probably about the same.