fix: Avoid re-raising exception that occur in then callbacks.

[dylanahsmith]

@eapache for review

Problem

Exceptions that were occurring in .then blocks were being re-raised, rather than just causing the returned promise to be rejected. If .then is called on a promise multiple times and an exception occurs in one of the callbacks, then that would prevent additional callbacks from being called.

This behaviour does not appear to be part of the promise spec, and a test with node shows this behaviour isn't consistent with javascript:

var resolve = null;
var promise = new Promise(function(resolveFn, rejectFn) {
    resolve = resolveFn;
});
promise.then(function(value) { console.log("first then"); })
promise.then(function(value) { throw "then error" })
promise.then(function(value) { console.log("last then"); })

try {
    resolve("value")
} catch(err) {
    console.log("caught:", err)
}

outputs

first then
last then

Solution

Remove the raise in the exception handler, fix the existing tests that asserted this behaviour, and add a new test to make sure the exception doesn't prevent additional callbacks from being called.

[eapache]

LGTM