Fix warnings in docs image generator build

Print a warning on system call failures

git-svn-id: http://svn.osgeo.org/postgis/trunk@12937 b70326c6-7e19-0410-871a-916f4a2858ee
Fix typo in DropOverviewConstraints doc

git-svn-id: http://svn.osgeo.org/postgis/trunk@12940 b70326c6-7e19-0410-871a-916f4a2858ee