RNG outputs should be validated

[joeytakeda]

Per TEIC/TEI#2538, as there may be some constructs that break RNG (but are near impossible to validate in ODD), it would be useful to ensure that the RNG file is valid and works before outputting it to the user.

@sydb suggested that the build process first checks that the RNG works, reads any error messages, and, if there are any, outputs those to the user (perhaps catching these errors in some way to give a more useful explanation) and then then deletes the temporary file.

I would also suggest that we allow some sort of switch (or re-use an existing one, like --debug) so that the temporary file is retained so the user can inspect that broken file, if they'd like.