On volumes failing scrubs, the detail output can get very verbose, which makes
reading results difficult. Most users won't care about this information to
begin with - just whether or not volumes pass scrub tests.
This MR gates the display of scrub result details behind a `--details` flag.
* Give the `ScrubVolume()` RPC an option to flag found broken volumes as read-only.
Also exposes this option in the shell `volume.scrub` command.
* Remove redundant test in `TestVolumeMarkReadonlyWritableErrorPaths`.
417051bb slightly rearranges the logic for `VolumeMarkReadonly()` and `VolumeMarkWritable()`,
so calling them for invalid volume IDs will actually yield that error, instead of checking
maintnenance mode first.