Groovydoc should set exit code on errors

Whenever the error occurs, GroovyDoc just reports it:

ERROR [org.codehaus.groovy.tools.groovydoc.GroovyRootDocBuilder] ignored due to RecognitionException: a/Test.java [expecting EOF, found '}']

and exits with success.

Whenever error occurs, GroovyDoc should set exit code so that whoever called Groovydoc could handle the failure, e.g. stop the build.