Add tracking of exception stack

[Seelengrab]

Since Julia 1.1, throw in a catch block will preserve the original error in an exception stack. Currently, Supposition.jl only looks at the topmost error in that stack, but it'd be good to attempt to shrink the total number of nested exceptions as well.

The place to change this initially is here:

Supposition.jl/src/teststate.jl

Lines 36 to 49 in d8540ac

	interesting, threw = try
	@with CURRENT_TESTCASE => tc begin
	ts.is_interesting(tc)
	end, nothing
	catch e
	# Interrupts are an abort signal, so rethrow
	e isa InterruptException && rethrow()
	# UndefVarError are a programmer error, so rethrow
	e isa UndefVarError && rethrow()
	# These are wanted rejections
	e isa TestException && return (false, false)
	# true errors are always interesting
	true, Some((e, stacktrace(catch_backtrace())))
	end

which would need to be changed from only looking at the current backtrace to looking at current_exceptions.

Further, this portion of code further down will also have to be changed, to take into account that there's multiple exception traces:

Supposition.jl/src/teststate.jl

Lines 92 to 119 in d8540ac

	if isnothing(ts.target_err)
	# we haven't had an error so far, but have we hit one now?
	if !isnothing(threw)
	err, trace = @something threw
	len = find_user_stack_depth(trace)
	ts.target_err = Some((err, trace, len, Attempt(copy(tc.choices), tc.generation, tc.max_generation)))
	return (true, true)
	end
	else # we already had an error - did we hit the same one?
	# we didn't throw, so this is strictly less interesting
	isnothing(threw) && return (false, false)
	err, trace = @something threw
	old_err, old_trace, old_len, old_attempt = @something ts.target_err
	old_frame = first(old_trace)
	frame = first(trace)
	# if the error isn't the same, it can't possibly be better
	if !(err == old_err && frame == old_frame)
	@warn "Encountered an error, but it was different from the previously seen one - Ignoring!" Error=err Location=frame
	return (false, false)
	end
	was_more_interesting = true
	len = find_user_stack_depth(trace)
	was_better |= len < old_len || (len == old_len && tc.choices < old_attempt.choices)
	if was_better
	ts.target_err = Some((err, trace, len, Attempt(copy(tc.choices), tc.generation, tc.max_generation)))
	end
	end

in particular, the way len is calculated needs to be adjusted so that it takes the total length of all nested exceptions into account. It should likely return a tuple of the number of nested exceptions and the total length.