Fixes issue Issue #229:

- A callback can be registered by using options.errorHandler
- If no errorHandler is specified an alert is triggered
- Howto use it:

var errorHandler = {};
errorHandler.login = function(e) { alert("your callback"); };
errorHandler.logout = function(e) { alert("your callback"); };
errorHandler.loginStatus = function(e) { alert("your callback"); };
errorHandler.dialogStatus = function(e) { alert("your callback"); };

var opts = {};
opts.errorHandler = errorHandler;

FB.login(callback, opts);