Change StubBroker.join() parameter fail_fast to default to True

[LincolnPuzey]

When fail_fast=True (now the default) and any message gets dead-lettered during the join, then an exception will be raised.

Resolves #739

[LincolnPuzey]

@Bogdanp @synweap15 @karolinepauls This change causes causes 26 of our tests to fail.
I'll need to fix them by setting fail_fast=False or catching the exceptions being raised.

But I want to double check we think the developer experience improvement here is worth the similar disruption that is likely in downstream projects.

[synweap15]

@LincolnPuzey it's a good direction given we're aiming at a major version bump. We'd be consistent, I'd say it's worth it.

It would be a good reference for developers if the tests caught the exception + at least one test that shows how fail_fast=False behaves.

[LincolnPuzey]

@synweap15 I'm thinking about projects with a large test suite that might find changing to the new behavior difficult.

How about we add a StubBroker option that controls the fail_fast default?
I've added commit a700ba7 to show what I mean.

That way large projects can do StubBroker(fail_fast_default=False) (presumably in a small number of places (e.g. pytest fixture)), if they want to upgrade to v2, but updating all their tests straight away is too much work.

[Bogdanp]

How about we add a StubBroker option that controls the fail_fast default?

Sounds like a good idea to me.