Slightly improve handling of kwcall definition lookup failure

[serenity4]

As part of Cthulhu tests, I caught an error where we try to get the definition of kwcall itself from https://github.com/JuliaLang/julia/blob/12f7bb52e5714c577189665a05606e3764f333cf/base/boot.jl#L366, and our handling fails thinking that it's a "wrapping" kwcall (and not an actual definition).

I don't think we can support parsing that well, so I just disabled that particular test in Cthulhu, but the failure mode was a bit obscure: it complained in our check T <: Function that T must be a type, and was Vararg, which this PR tries to make a bit clearer.

A very minor improvement, but I thought it wouldn't hurt to improve things once I figured out the reason for the subtyping check failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.10%. Comparing base (c0ca2ff) to head (5679ae9).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #147       +/-   ##
===========================================
+ Coverage   69.60%   89.10%   +19.49%     
===========================================
  Files           3        3               
  Lines         408      413        +5     
===========================================
+ Hits          284      368       +84     
+ Misses        124       45       -79     

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[timholy]

Thanks! Would it be difficult to add a test? Assuming it now gives a less confusing error, could we, e.g., @test_throws "some message" ...?

[serenity4]

Sure! I actually had one case in hand already when making that PR, don't know why I didn't think of adding it straight away :)

[timholy]

Any objections if I transfer this repo to JuliaDebug? Or some other org?

[serenity4]

None at all!