fix(Observable): Another attempt at fixing error rethrows in v5

[jayphelps]

This is an attempt at fixing the rethrow issues in #2813. Some of them have been fixed by previous PRs, but other cases still remain. So far every one I've seen has been fixed by this PR.

I'll be totally honest though, why this works isn't 100% clear. The syncErrorThrowable stuff is pretty hairy. I tried to map out what syncErrorThrowable is being used for, when its set correctly, when its not, etc and noticed a pattern that errors were going down the _trySubscribe when there was a this.source but that always expected syncErrorThrowable = true otherwise the condition right after wouldn't do the rethrowing.

Mostly what's not clear is why this only happens with Subject, timer, interval, etc combined with a nested Observable via merge/concat/etc. Seemed like it might be related to the fact that these override _subscribe but others do too and I couldn't find a pattern otherwise.

The minimum reproduction here is, somewhat strangely, the inverse of what most often the problem seems to be. I tried to also find a minimum reproduction for what I think is the issue most often found (swallowing instead of rethrowing) I couldn't. The closest I could find was this:

const input = new Subject();
const result = input
  .mergeMapTo(Observable.of(1).mapTo(1));

result.subscribe(value => {
  throw 'error!';
});
        
try {
  input.next(1);
} catch (e) {
  console.log('does not get caught, without this PR');
}

I'm hoping that either this is good enough or perhaps this will help someone get a more clear understanding of the problem.

Fixes #3183, #2813

[rxjs-bot]

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[coveralls]

Coverage increased (+0.0004%) to 97.4% when pulling 541b49d on jayphelps:errors into 520b06a on ReactiveX:stable.

[jayphelps]

this one in particular I'm weary about, but for reasons unknown to me it's the only way the other change works in every case I test.

[jayphelps]

this test fails without the two other changes, so I made it as a separate commit to make it a bit easier for someone to confirm if they want

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.