fastboot: Change -w to format after the erase of userdata & cache

If the bootloader doesn't support formatting of those partitions
(either because it doesn't support the getvar commands needed or
the partition type is not supported), the errors are printed but
doesn't halt processing of subsequent commands.

Change-Id: I816ac2e5e7593846fcb4fd39c793a8dbdd996f6f
Signed-off-by: Mike J. Chen <mjchen@google.com>