var save = ( sforceClient.update([opportunity]) )[0];    
        
if (save.success == true) {
    /*
        ....
    */        
} else {
    window.alert("Rats! There was a problem and I think the problem is: " + save.errors[0].message);
    window.location.replace("/" + opportunityId);
}   

Is save.errors.message all I can have? Due to the accessibility issue, I may need to know the error in detail, say the fields which are hidden to current user.

as title. There's no such a tag available in it :(

var opportunity = new Sforce.Dynabean("Opportunity");

opportunity.set(.....
.....

if (saveResult[0].success == true) {
            output += "Opportunity has been created, ID is: " + saveResult[0].Id + "."; 
            document.getElementById("output").innerHTML = output;
}

But I got this eventually:
    "Opportunity has been created, ID is: undenfined."

var opportunity = new Sforce.Dynabean("Opportunity");

opportunity.set(.....
.....

if (saveResult[0].success == true) {
            output += "Opportunity has been created, ID is: " + saveResult[0].Id + "."; 
            document.getElementById("output").innerHTML = output;
}

But I got this eventually:
    "Opportunity has been created, ID is: undenfined."