Port Base.current_exceptions() to older Julia versions

[c42f]

See JuliaLang/julia#29901

This is limited to julia-1.1 and above because earlier versions don't
have the necessary runtime library support (Base.catch_stack() etc).

[codecov]

Codecov Report

Merging #746 (d5a5199) into master (ad9a668) will increase coverage by 0.28%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
+ Coverage   81.71%   81.99%   +0.28%     
==========================================
  Files           4        4              
  Lines         536      561      +25     
==========================================
+ Hits          438      460      +22     
- Misses         98      101       +3     

Impacted Files	Coverage Δ
src/Compat.jl	82.02% <88.00%> (+0.32%)	⬆️

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[c42f]

Failing tests on nightly are #745

[martinholters]

With a README entry and after a rebase, this looks good to me.

[c42f]

I added the readme entry and rebased.

Also I've added julia-1.0 compatibility (in that case, the best we can do is just return the current exception as a single-element stack)

[martinholters]

LGTM superficially, but you probably know better than me whether this does the right thing.

But maybe we should mention the limited functionality on Julia 1.0 in the README?

[c42f]

whether this does the right thing.

It should do the right thing to the extent that the right thing can be done at all in julia 1.0 (there were some nasty bugs with exceptions and backtraces going missing completely in julia-1.0 due to task switching. But Compat can't work around that.)

I figured it was better to have the API with only approximate runtime compatibility in 1.0, than to not have the API at all — it's a lot easier for users if they can at least rely on the API being present on all versions supported by Compat.

[martinholters]

I don't disagree. Only question: Merge as is or mention the limitation re: Julia 1.0 in the README?

[c42f]

Yeah, it's worth mentioning the Julia-1.0 limitations in the readme. I did that.

[martinholters]

Alright, there was plenty of time for others to chime in on this, bu no one did, so here we go...