Add MOJO_PROMISE_DEBUG environment variable

[kraih]

I'm not 100% sure about this feature, but it seems pretty useful for locating your unhandled rejected promises.

Instead of

Unhandled rejected promise: branches3 at /Users/sri/repo/mojo/lib/Mojo/Reactor/Poll.pm line 131.

you would see

Unhandled rejected promise: branches3 at /Users/sri/repo/mojo/lib/Mojo/Reactor/Poll.pm line 131.
-- Destroyed promise
Traceback (most recent call first):
  File "/Users/sri/repo/mojo/lib/Mojo/Promise.pm", line 55, in "Mojo::Promise"
  File "/Users/sri/repo/mojo/lib/Mojo/Promise.pm", line 94, in "Mojo::Promise"
  File "t/mojo/promise.t", line 444, in "main"
  File "/Users/sri/perl5/perlbrew/perls/perl-5.32.0/lib/site_perl/5.32.0/Test/Builder.pm", line 374, in "Test::Builder"
  File "/Users/sri/perl5/perlbrew/perls/perl-5.32.0/lib/site_perl/5.32.0/Test/Builder.pm", line 374, in "Test::Builder"
  File "/Users/sri/perl5/perlbrew/perls/perl-5.32.0/lib/site_perl/5.32.0/Test/More.pm", line 809, in "Test::More"
  File "t/mojo/promise.t", line 450, in "main"

is you set MOJO_PROMISE_DEBUG=1. Which made is very easy to conclude that line 444 is the real source of the forgotten promise.

[jberger]

I have always wanted something like this. When it was callbacks I tried several times to come up with basically this same thing but always got tripped up, mostly because I didn't have a good place to store info. With an object like Promise its much easier to tuck something in there. I haven't reviewed the code but I'm 100% on board with the concept!

[christopherraa]

👍 from me. Unhandled promises has tripped me up so many times. This is a great addition.