vfat, ext2: print error message when size is zero

[gportay]

The `size' property is mandatory and cannot be 0 for vfat and ext file systems.

Print a message that is more helpful than:

genext2fs: too few blocks. Note: options have changed, see --help or the man page.
ext2(test.ext2): failed to generate test.ext2

and

mkdosfs: unable to discover size of images/test.vfat
vfat(test.vfat): failed to generate test.vfat

Signed-off-by: Gaël PORTAY gael.portay@savoirfairelinux.com