netcdf driver: only call nc_close on a valid netcdf id when closing dataset ; reduce verbosity

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26151 f0d54148-0727-0410-94bb-9a71ac55c965
suppress error messages when opening /vsimem dataset in HTTP driver - not all drivers support this

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26152 f0d54148-0727-0410-94bb-9a71ac55c965