Annotation processor exceptions in forked Javac are ignored

A POM with an annotation processor missing a dependency will fail to compile, but maven-compiler-plugin will not print the exception.

See https://github.com/codehaus-plexus/plexus-compiler/issues/106

Reproducer here: https://github.com/danberindei/maven-compiler-test/tree/annotation_processor_exception