Actually, you can throw exceptions from within .then just fine and they will be always bubbled. You just cant go

  function g() {
    return somePromise.then(function() { 
      setTimeout(function() {
        throw new Error();
      }, 500);
    });
  }
  g(); // crash

But at that point, you're not using promises anymore so all bets are off.

However if you use a promise-based timeout too:

  function f() {
    return somePromise.then(function() {
      return Q.delay(1000).then(function() {
        throw new Error("Oops");
      });
    });
  }
  f().catch(function(e) {
    console.log(e.message); // logs Ooops.
  });

No problem bubbling that.