Make the error message about wrong calculated size in twkb a little more verbose.

git-svn-id: http://svn.osgeo.org/postgis/trunk@12321 b70326c6-7e19-0410-871a-916f4a2858ee
Add a twkb regression test more

git-svn-id: http://svn.osgeo.org/postgis/trunk@12322 b70326c6-7e19-0410-871a-916f4a2858ee