Selecting a bad block scheduler gives a cryptic failure

[jlowellwofford]

By default, the block scheduler is "noop". If an invalid scheduler is written, the only notification of that failure is an unspecified "short write" error, which causes an over-all mount failure.

It would be nice (1) to have a more descriptive failure on writing the block scheduler; and, (2) have a more likely default ("mq-deadline"?).