The output files of the task (which defaults to false) will differ vs. what is generated at runtime with the web.xml 'mappedfile' parameter set true. I just did a quick check and 'die' seemed to be the only other option that was missing that might make sense in the task. These also occur in all versions of Tomcat and might be ported to 5.x.
I've implemented setMappedFile() in 8.0.x and it will be included in 8.0.13 onwards. die level is specific the calling JspC from the command line and is not used when using the Ant task so I see no need to provided a setDieLevel method. There has been little demand for this enhancement request so I do not intend to back-port it to 7.0.x or 6.0.x.
I applied the fix to Tomcat 7 in r1624249 (it will be in 7.0.56) and proposed for Tomcat 6.
This has been fixed in 6.0.x and will be included in 6.0.42 onwards.