Show full stacktrace when Channel Task throws

[non-Jedi]

Not sure if this counts as a feature or a bug-fix. This was a major
pain-point for a user on gitter which seemed an easy fix once I
understood the system.

This reuses the machinery that gives good stacktraces when a user
calls wait on a Task. Since bind internally uses _wait2, this
machinery is bypassed currently.

Currently, julia throws an uninformative stacktrace in this situation:

julia> c = Channel(_ -> error("foo"))
Channel{Any}(sz_max:0,sz_curr:0)

julia> for i in c end
ERROR: foo
Stacktrace:
 [1] iterate(::Channel{Any}) at ./channels.jl:459
 [2] top-level scope at ./REPL[2]:1

With this change, the stacktrace is much more informative:

julia> for i in c end
ERROR: TaskFailedException:
foo
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] (::var"#1#2")(::Channel{Any}) at ./REPL[4]:1
 [3] (::Base.var"#652#653"{var"#1#2",Channel{Any}})() at ./channels.jl:129
Stacktrace:
 [1] check_channel_state at ./channels.jl:167 [inlined]
 [2] take_unbuffered(::Channel{Any}) at ./channels.jl:405
 [3] take! at ./channels.jl:383 [inlined]
 [4] iterate(::Channel{Any}, ::Nothing) at ./channels.jl:449
 [5] iterate(::Channel{Any}) at ./channels.jl:448
 [6] top-level scope at ./REPL[5]:1

[non-Jedi]

Any comments on this or things I need to fix? If this isn't considered too breaking of a change, I believe this is ready to merge.

[non-Jedi]

Bumping this per fredrikekre's post on discourse in case it can make it into 1.5. I understand if it can't.

[StefanKarpinski]

This can still make it into 1.5. Tagging for discussion on the triage call today.

[non-Jedi]

I just noticed that I missed a braindead simplification in this PR and pushed e17696e to make this code actually make sense to people unaware of its history.

[StefanKarpinski]

I don't see a PR for that yet. Am I missing it or is it yet to be opened?

[non-Jedi]

I pushed it to the branch for this PR but can open a new one. Looks like CI doesn't get run if you push to a branch after it's merged, so I'll go ahead and open a new PR.

EDIT: #35673

[NHDaly]

Thanks for this, @non-Jedi! :) This is great - we just noticed this was missing in 1.4 and started drafting a fix before noticing you'd already fixed it on 1.5 😊 👏 👍

[non-Jedi]

Glad this was helpful to y'all. :)