Changed GradeDisplay::SetGrade to print errors instead of segfaulting on unloaded actors.

src/GradeDisplay.cpp

@@ -25,12 +25,19 @@ void GradeDisplay::Load( RString sMetricsGroup )
 
 void GradeDisplay::SetGrade( Grade grade )
 {
-	int i = 0;
+	size_t i = 0;
 	FOREACH_PossibleGrade( g )
+	{
+		if(i >= m_vSpr.size())
+		{
+			LuaHelpers::ReportScriptError("GradeDisplay:SetGrade: No actor loaded for grade " + GradeToString(g));
+		}
+		else
 		{
 			m_vSpr[i]->SetVisible( g == grade );
 			i++;
 		}
+	}
 }
 
 // lua start