Improve error message for PRNG seeding failure

Karl had a machine where /dev/urandom had changed to a regular file, and noted that we gave a cryptic error message.  This patch gives a better one, although it's not super-specific as talking about /dev/urandom would be wrong on Windows.

I didn't mark this for pullup since it's a pretty unusual case, but I did give it a ticket since it's a user-visible behavior change.