Show exception with stack trace on send_message error

[yha]

Helps with JuliaGizmos/Interact.jl#290.
It doesn't solve that issue yet, since errors are still swallowed if produced in the initial position of the @manipulate slider.

[twavv]

Thanks for the PR.

Can we instead just include this in the @error invocation?

trace = sprint(io -> showerror(io, ex, stacktrace(catch_backtrace())))
@error(
    ...,
    trace,
)

[yha]

I tried that initially, but passing the trace as a second argument like this shows it as an escaped string with \ns and no newlines, which is pretty hard to read.
The new commit adds the trace into the first argument, which seems to work fine.

[pfitzseb]

The "right way" to do this is to pass an (exception, backtrace) tuple to @error:

julia> try
       0//0
       catch e
       @error("0//0", exception=(e, catch_backtrace()))
       end
┌ Error: 0//0
│   exception =
│    ArgumentError: invalid rational: zero(Int64)//zero(Int64)
│    Stacktrace:
│     [1] __throw_rational_argerror(::Type) at ./rational.jl:19
│     [2] Rational at ./rational.jl:14 [inlined]
│     [3] Rational at ./rational.jl:21 [inlined]
│     [4] //(::Int64, ::Int64) at ./rational.jl:44
│     [5] top-level scope at none:2

[yha]

Right, I somehow missed that going through the @error docs. Updated the PR.

[twavv]

LGTM.

TravisCI failures are seemingly unrelated (though should be fixed).