_createError = function(oRequest, sUserMessage) {
		var oError = new Error(oRequest.statusText);
		oError.status = oRequest.status;
		oError.userMessage = sUserMessage;
		return oError;
	}
Creates <code>Error<code> object from <code>XMLHttpRequest<code> and an additional message for end user

@param {XMLHttpRequest} oRequest <code>XMLHttpRequest<code>
@param {string} sUserMessage Message which can be displayed to end user
@returns {object} <code>Error<code> object