Check for another kind of definition fail.

We can't be both legacy and modern at the same time.

sc2/src/uqm/planets/generate-base.cpp

@@ -247,6 +247,16 @@ namespace Planets  ::detail::  Generate_m
 				}
 
 				std::cerr << "  yes" << std::endl;
+
+				if( star.Index )
+				{
+					const std::string mesg= "Solar system with modern generator (`"s
+						+ star.supplement + "`) has a legacy index!";
+
+					std::cerr << mesg << std::endl;
+
+					throw std::runtime_error{ mesg };
+				}
 			}
 		}
 	}