jst-lower: Trap add_log_message to print to somewhere we can see

jst-lower: Trap add_log_message to print to somewhere we can see

Otherwise, we just get an exception printed by nodejs which doesn't
actually include the message.

What we do here is print it to console.log where it will end up in
stderr, and *also* print a version to the machine-readable output
parsed by jst-lower.rs.

Currently jst-lower.rs bails on seeing it, which means these kind of
log messages are still fatal errors.  This is what we want - we don't
have any test cases that expect them.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>