replace a few assertions with error codes based on netCDF https://github.com/Unidata/netcdf-c/commit/4a90e9a8883e3f10118c770ea0282317a1de0c3d

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1579 c368ceb8-fd28-0410-a457-e46ee1d7cd4a