Improve the documentation for the compiler attribute in the Jasper how-to

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1437640 13f79535-47bb-0310-9956-ffa450edef68
Improve error message by including the name of the file when the java file generated from a tag file cannot be compiled.
Based on a patch by Sheldon Shao.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1437656 13f79535-47bb-0310-9956-ffa450edef68