bug#4807: 23.1.50; Unhelpful user visible type error in transpose-lines

[Geoff Gole]

With an argument of 0, transpose-lines will swap the lines that point
and mark are in. In a fresh buffer in which mark has not yet been set
this will result in a type error:

   (wrong-type-argument integer-or-marker-p nil)

This isn't very helpful to the user. exchange-point-and-mark fails in
the same conditions with the much clearer phrasing "No mark set in
this buffer", perhaps transpose-lines should copy that.