#316 Add warning for plugin ant builds if xena.jar not present

[Terry O'Neill]

The ant builds of plugins depend on the presence of a "xena.jar" file under the "../../xena/dist" directory with reference to the build.xml file in an individual plugin. When ant is run to use this build.xml file without the presence of xena.jar then compile warnings are given about imports that could not be found. It would be better if these ant build.xml files were setup with a "fail" section to give a more user friendly error message that the xena.jar file is missing and that ant must be run in the base xena directory to generate it.