RAID: Allow remove/replace of sub-LVs composed of error segments.

RAID:  Allow remove/replace of sub-LVs composed of error segments.

When a device fails, we may wish to replace those segments with an
error segment.  (Like when a 'vgreduce --removemissing' removes a
failed device that happens to be a RAID image/meta.)  We are then left
with images that we will eventually want to remove or replace.

This patch allows us to pull out these virtual "error" sub-LVs.  This
allows a user to 'lvconvert -m -1 vg/lv' to extract the bad sub-LVs.
Sub-LVs with error segments are considered for extraction before other
possible devices so that good devices are not accidentally removed.

This patch also adds the ability to replace RAID images that contain error
segments.  The user will still be unable to run 'lvconvert --replace'
because there is no way to address the 'error' segment (i.e. no PV
that it is associated with).  However, 'lvconvert --repair' can be
used to replace the image's error segment with a new PV.  This is also
the most appropriate way to do it, since the LV will continue to be
reported as 'partial'.