Backport r534 to XIOS 1.0: Add an error message when the timestep was not configured by the user.

Previously no error message was displayed but the output file(s) was not written.

Also improve the error message when the calendar type or the start date is missing.

git-svn-id: http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/branchs/xios-1.0@535 d613d254-d18a-4304-b296-5125d4c4d670