missing processingEnv assignment in overridden init results in too minimal exception notice.

When @SupportedSourceVersion is not used, if user overrides the init method and does not assign to processingEnv, a null pointer exception results when processor is run. The error message is kind of minimal

error: Exception thrown while constructing Processor object: java.lang.NullPointerExceptioN

and give little indication of what's wrong. Maybe more of the stack track may help(?).