sbin/hammer: Change fprintf/exit to err variants [1/2]

sbin/hammer: Change fprintf/exit to err variants [1/2]

Change
fprintf(stderr, ...); exit(1);
and
perror(...); exit(1);
to
err(1, ...) or errx(1, ...);
where possible for consistency.

In test_volume(), if open(2)/pread(2) failed, err(1) without
scanning rest of the volumes. This function itself is redundant
anyway as mentioned in 1e297b34, so no one cares.

Other than that this is just conversion to err(3) variants.
Messages themselves are not changed, except for removing
strerror(errno) for err(3), and removing trailing \n,
though err variants add program name in the messages.

err(3) and variants are non standard BSD functions, but HAMMER
userspace has been using BSD stuff aside from the existing
err/errx. Also note that err(3) is available in Linux as well.