When you are doing client/server communication with ExtJS you probably run into the problem that you want to handle server side errors in a generic way. A solution that I found is to override the handleFailure function in the Ext.data.Connection class:

Ext.data.Connection.prototype._handleFailure = Ext.data.Connection.prototype.handleFailure;
Ext.data.Connection.prototype.handleFailure = function(response, e) {
	var errorText = Ext.DomQuery.selectValue("Reason/Text", response.responseXML, "Unknown Error");
	Ext.Msg.alert('Error', errorText);
	Ext.data.Connection.prototype._handleFailure.call(this, response, e);

This handler simply is called whenever a server side failure occurs. As the Connection class handles all communication this can happen also in Ext.Ajax or Ext.data.HttpProxy. In this example the error message to be displayed is retrieved from a server generated XML with the actual text being located at XPath Reason/Text.

So a server side error message might look like this:

<text lang="en-US">Some error from the server-side</text>