Simplify Redfish error message property interface

The error message code used a json_pointer object which must
begin with a '/' character and had to be sent as an extra
parameter.  This change simplifies the interface by using a
string so there doesn't have to be a '/'.  This allowed the
same property argument passed for the message to be used
for the property field path.

Tested: Sent an error with a property and verified that it is
correctly displayed.

Change-Id: I0571e2eee627cedf29d751588a4f1bceee66f084
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>