Define custom message for validators

[newmanw]

Currently you can only define a custom type for validators. This makes it extremely cumbersome to try and generate an error message that is suitable to return to a client. I.E.

User.schema.path('username').validate(function(value, done) {
User.findOne({username: value}, function(err, user) {
if (err) return done(false);
if (user) return done(false);

done(true);

});
}, "exists");

I have to parse out the error msg:

for (error in err.errors) {
msg += "Error creating/updating user: " + error.path + " " + error.type;
}

Would be nice to add a custom message to validator.
User.schema.path('username').validate(function(value, done) {
User.findOne({username: value}, function(err, user) {
if (err) return done(false);
if (user) return done(false);

done(true);

});
}, "exists", "user name already exists");

[aheckmann]

duplicate of #747 which just landed on master.