Fix grow-disk handling of invalid units

The reason why grow-disk was doing:

$ gnt-instance grow-disk instance3 0 -64
Unhandled Ganeti error: Invalid format

Is because it does it's own ParseUnit call, and doesn't transform that
into a nicer message.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>