Convey need for snapshot-merge target in lvconvert error message and man

Convey need for snapshot-merge target in lvconvert error message and man
page.

Add ->target_name to segtype_handler to allow a more specific target
name to be returned based on the state of the segment.

Result of trying to merge a snapshot using a kernel that doesn't have
the snapshot-merge target:

Before:
# lvconvert --merge vg/snap
  Can't expand LV lv: snapshot target support missing from kernel?
  Failed to suspend origin lv

After:
# lvconvert --merge vg/snap
  Can't process LV lv: snapshot-merge target support missing from kernel?
  Failed to suspend origin lv
  Unable to merge LV "snap" into it's origin.