sbin/hammer: Fix error message on pfs open/ioctl failure

sbin/hammer: Fix error message on pfs open/ioctl failure

- "Not a HAMMER root" doesn't seem to be a good error message
  when getpfs() or ioctl(HAMMERIOC_GET_PSEUDOFS) has failed.

- getpfs() returning negative indicates open(2) has failed.
  ioctl(HAMMERIOC_GET_PSEUDOFS) returning negative (particularly
  when caller has set pfs->pfs_id to -1 before ioctl) usually
  indicates the fd has nothing to do with the existing valid pfs.
  It makes better sense with a message like "Invalid PFS path %s"
  or something.