skip failed guestfs-os-inspect mounts and try to continue

Some custom guests may have fstab content with "nofail" as a mount
option.  For example, images built for EC2 with ephemeral mappings.
The guestfs inspection code reports these mounts and they fail if
we try to mount them during normal Oz operations like ICICLE
generation.  This patch allows things to continue.  Profound issues
in the output image should trigger errors later during the process.