reported by Damien Lebrun-Grandie dalg24 =at= gmail =dot= com

With the current version of GSL (without the patch), the user do not get a chance to check the return status of gsl_interp2d_eval_e() because GSL_ERROR() gets called direclty instead of returning an error code as advertised in the documentation.

With the changes I propose, the domain error will still be caught by macro DISCARD_STATUS() that checks the return code of gsl_interp2d_eval_e() in gsl_interp2d_eval() but the intended behavior for gsl_interp2d_eval_e() will be achieved.  The only downside I see with this patch is that the user now gets an out-of-domain error but he is not told anymore whether the error was caused by the x or the y coordinate.