Fail AutoValueExtensionsException with Error, not Warning

[mseele]

I've experienced an exception of a AutoValue Extension:

rharter/auto-value-gson#258

It was pretty hard to find the problem because com.google.auto.value.processor.AutoValueProcessor.init(ProcessingEnvironment) only reports a warning via errorReporter().reportWarning instead of an error. Maven did not break the build because it is just a warning and not an error. I would expect that the AutoValue compilation failed when an AutoValueExtension throws an error.