If class writing fails, attempt diagnoses

[som-snytt]

If class writing throws, detect certain failure modes (writing strings).

This is just a start.

[som-snytt]

I didn't compile this under -Wvalue-discard.

Also neglected to check the messaging from javac.

[lrytz]

This looks good to me, thanks! I'm not so sure what we have and what other failures are maybe somewhat common, eg with generated code. Either way, good progress.