Pass location with errors in codegen to have markers in editors in Eclipse

Tried a few things in Eclipse (static `@Provides` method, private `@Provides` method, abstract module class, class with `@Provides` but without `Module`) and saw the markers.

I think I found a bug in `FullGraphProcessor` during my tests, where the included modules are assumed to have a `@Module` annotation, which can lead to an NPE if it's not the case.

Fixes issue #110