Fix possibly incorrect error messages in case of NetCDF failure.

Some return statements were misplaced which could lead to unpredictable behavior.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@578 d613d254-d18a-4304-b296-5125d4c4d670