emit message for unknown GDAL drivers

git-svn-id: http://svn.osgeo.org/postgis/trunk@12539 b70326c6-7e19-0410-871a-916f4a2858ee
disable one unit test

git-svn-id: http://svn.osgeo.org/postgis/trunk@12540 b70326c6-7e19-0410-871a-916f4a2858ee
tweak messages

git-svn-id: http://svn.osgeo.org/postgis/trunk@12541 b70326c6-7e19-0410-871a-916f4a2858ee