Error Reporting?

So I've inherited a half-finished custom plugin which allows users to share directories between themselves (Share Resource). Errors from it come back blank, and about all I've been able to discern so far is that the xml.checkError() is returning true. When I attempt to use getErrorNumber(), though, I get a -1. Is it just failing to find any XML at all, or is there a specific -1 error code I'm missing?

sample check below:

api.connector.sendCommand('ShareResource', funcParams, function(xml) {
if (xml.checkError()) {
var customError = xml.getErrorNumber();
alert(customError);
return;
}
else {
...