Much simpler handling of timestamp errors. Return an error to the user

Much simpler handling of timestamp errors.  Return an error to the user
when any of the time stamp values are invalid.  This is similar to
RRDTool's normal behavior.  Removed the complex logic previously used to
return error codes to the user.

This solves a bug where non-advancing timestamps could have produced
incorrect error output during "BATCH" mode.  The bug was cause by using
the sock->wbuf pointer for the error output. -- kevin brintnall

git-svn-id: svn://svn.oetiker.ch/rrdtool/trunk/program@1648 a5681a0c-68f1-0310-ab6d-d61299d08faa