How to capture reject on Q.all() when querying mongoose? - promise

Question

I finally figured out how properly to use Q.all() in my code and it works as expected, but I don't know how to detect the reject if error comes from database in my specific code. I googled a lot but the problem is that in this particular case I can't relate the information I find by google to my own problem! Now with code, I have:
function username(user) {
var deferred = Q.defer();
var queryu = User.find();
queryu.where({_id: user});
queryu.exec(function(err, results) {
if (err) { //system level error
deferred.reject(err);
} else {
var nameAndFB = extractinfo(results[0]);
deferred.resolve(nameAndFB);
}
});
return deferred.promise;
}
later, I have another method that uses this one:
function masterUserObj(user, curstate) {
var p1 = username(user);
var p2 = getState(curstate);
return Q.spread([p1, p2], function(userinfo, pairstate) {
var obj1 = {};
obj1.username = userinfo[0];
obj1.fbid = userinfo[1];
obj1.idprovider = userinfo[2];
obj1.state = pairstate;
return obj1;
});
}
finally, a for loop puts all above to use:
function exposePairs(results, res) {
var plist = [];
for (var i = 0, m = results.length; i < m; i++) {
plist[i] = masterUserObj(results[i].user, results[i].state);
}
Q.all(plist).then(function(theArr) {
return res.jsonp({pairs: theArr});
});
}
Code works, but I don't know where and how best to detect the reject case: deferred.reject(err);
Mainly I'm confused because I just learned to put promises in use together with a loop.
Please point me to the best practices.
Edit:
Also, please comment on code if I should use .done() after the final Q.all() or it is not necessary here.