#2646: Replace error/warning messages by CCTK_ERROR/CCTK_WARN in IllinoisGRMHD

Commits f3202a7 and 78143cb to the master branch of wvuthorns provide updates to IllinoisGRMHD that have been previously suggested. These are:

The sequence printf(error_message); exit(1) in harm_primitives_lowlevel.C of IllinoisGRMHD has been replaced by CCTK_VERROR(error_message);
A warning messaged printed using printf(...) in inlined_functions.C has been replaced by the appropriate call to CCTK_VWARN(CCTK_WARN_ALERT, ...).

Cheers,

Leo