Enable more compiler warnings and treat them as errors.

src/OptionRow.cpp

@@ -857,7 +857,7 @@ void OptionRow::Reload()
 		ImportOptions( vpns );
 		FOREACH_HumanPlayer( p )
 			AfterImportOptions( p );
-		// fall through
+		[[fallthrough]];
 	}
 
 	case RELOAD_CHANGED_ENABLED:
@@ -865,6 +865,9 @@ void OptionRow::Reload()
 		FOREACH_HumanPlayer( pn )
 			PositionUnderlines( pn );
 		break;
+
+	default:
+		break;
 	}
 
 	// TODO: Nothing uses this yet and it causes skips when changing options.