Fix for GROOVY-6386 - groovyc task throws ZipException when it encounters classpath resources that are not jars.

The javac task successfully ignores these resources.  As a result, adding groovyc compilation to existing build scripts that previously only used javac can introduce unexpected compilation failures.  The groovyc ant task does not output the offending classpath resource, making diagnosis more difficult.

The modification catches the exception and continues on, adding a warning log about the offending resource.