inspection: report guest inspection errors

Add an `error' flag to the inspection data for a guest and set it when
the inspection failed.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
details: show warning when guest inspection fails

If libguestfs fails to inspect the guest then show a warning.

Now the "Operating System" and "Applications" frames are visible
only when the guest inspection is available.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=755869

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>