Adding disk to mirror boot device fails without useful information if new disk is too small
When adding a disk to convert the boot device into a mirror, a disk that is too small causes a traceback that tells the user nothing:
[MiddlewareError: Command '('gpart', 'add', '-t', 'freebsd-zfs', '-I', '2', '-a', '4k', '-s', '16008040448B', 'da0')' returned non-zero exit status 1.]
Before calling gpart, this code should query the new device capacity. If the available capacity is too small to mirror the existing device, return an error message with that information, then stop.
The new 'Sandisk 16G USB' device only has 15.6 GiB available capacity. To mirror the boot device, at least 16.0 GiB is required. Operation aborted.