test: fat: add error-checking to non-contig test

Check the result code of all command that are executed. Without this,
if the fallocate invocation fails (this feature is not supported on ext3
filesystems for example) then a zero-length output file will be created,
and subsequent the mkfs and mount invocations will fail, which will cause
the subsequent dd invocation to attempt to fill up the host's entire free
disk space. That's not a nice user experience!

Related, if fallocate does fail, try to create the test disk image using
dd instead. That should work everywhere.

Fixes: 4a282742 ("test: fat: add test of non-contiguous file reads")
Signed-off-by: Stephen Warren <swarren@nvidia.com>