Add Exception Summaries

[msagarpatel]

This PR adds the ability to summarize the current exception. This is especially helpful in cases where there are large exceptions with multiple TaskFailedExceptions and CompositeExceptions wrapping multiple exceptions.

The interface is simple:

    summarize_current_exceptions(io::IO=Base.stderr)

Examples:

julia> begin
           try
               try
                   @sync begin
                       Threads.@spawn try
                           @assert false
                       catch
                           @assert 1 != 1
                       end
                       Threads.@spawn @assert 2 != 2
                   end
               catch
                   @assert 3 != 3
               end
           catch
               summarize_current_exceptions()
           end
       end

=== EXCEPTION SUMMARY ===

CompositeException (2 tasks):
 1. AssertionError: false
     [1] macro expansion
       @ ./REPL[14]:6 [inlined]

    which caused:
    AssertionError: 1 != 1
     [1] macro expansion
       @ ./REPL[14]:8 [inlined]
 --
 2. AssertionError: 2 != 2
     [1] (::var"#10#12")()
       @ Main ./threadingconstructs.jl:258

which caused:
AssertionError: 3 != 3
 [1] top-level scope
   @ REPL[14]:13

TODOs

• Test this in a larger codebase that can generate truly HUGE exceptions
• Unit tests (WIP already working on this)
• Seen set, for deduplication
• Colorize the exception, not the "caused by"
• Add examples to this description
• Add ability to pass arbitrary exception rather than only allow current exception

[NHDaly]

@msagarpatel:
Regarding the TODOs:

• Test this in a larger codebase that can generate truly HUGE exceptions
  • Good idea, but we could probably merge this PR as-is, and then iterate!
  • No need to let perfection be the enemy of the good.
• Unit tests (WIP already working on this)
  • I think the tests we have now seem decent! What else would you want to add?
• Seen set, for deduplication
  • I think we can do this in a follow-up. No need for it now.
• Colorize the exception, not the "caused by"
  • Meh.. I don't think this is easy, and i don't think it's worth doing.
  • Base doesn't color the exception names, and we're reusing their printing, so i don't think this is feasible, nor important.
• Add examples to this description
  • sure, sounds nice. I can copy in one of the examples I added as a test. Is that enough?
• Add ability to pass arbitrary exception rather than only allow current exception
  • Fine to leave this as a follow-up, or never do it. I don't think this really makes
    sense considering that the whole goal here is to print a whole exception stack.
  • With just a single exception, you could just call Base.display_error().

[NHDaly]

@msagarpatel: I think this PR LGTM!

I'll leave it open in case there's anything else you want to address, but this looks really great. :)

I made a few more improvements today, and I think this is ready to use! We should plug it in to RAICode, since it's not helping anyone just sitting here. 😊 We can always make more changes later, but i think this would already be really, really helpful.

[msagarpatel]

Thanks @NHDaly! I'm still going through your comments and changes. Gonna check out for the night, but I'm aiming to wrap this up tomorrow.