Support send_robust with tracebacks (similar to Django signals)

[arnavk]

This PR builds on top of #3 by @jwineinger.

• Support send_robust
• Add traceback to every exception returned in the response from send_robust

cc @jek

[arnavk]

cc @jek. Any thoughts?

[TheLazzziest]

Hi, @arnavk! Are you planning to continue working on this PR ?

[arnavk]

@TheLazzziest, unlikely. I've lost context on this, unfortunately! its been 5 years now 😅

[davidism]

Closing as this is very old and I'm not clear that it's needed.

With this PR there's no way to distinguish whether a receiver deliberately returned an exception or whether it failed. Admittedly uncommon, but it's meaningful, especially with static typing.

Also, it's not clear that "signals could fail" is a good thing. If you're not checking the return value (just sending and forgetting), you now are silently ignoring errors. Some of Flask's callbacks can't fail, so we just document "you should ensure your callback won't raise an exception", that seems appropriate advice for signals as well.