This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
Looks like there is no request attribute named "Error". Fix your code to either supply such an attribute, or fix the JSP code to handle the possibility of no "Error" attribute. (Or better yet, get rid of the server-side script in your JSP and make use of tag libraries and the expression language instead.)