Report which missing types caused a "not all of its dependencies could be resolved" error

[netdpb]

When an element cannot be processed because it depends on some other element that should be generated by a different annotation processor, and that dependency isn't resolved by the last round of processing, BasicAnnotationProcessor reports: " was unable to process because not all of its dependencies could be resolved. Check for compilation errors or a circular dependency with generated code."

It would be nice to report, when possible, exactly which types couldn't be resolved.

This is related to google/dagger#1381.