patch 9.0.1558: wrong error for unreachable code after :throw

Problem:    Wrong error for unreachable code after :throw.
Solution:   Adjust the error message.
diff --git a/src/vim9.h b/src/vim9.h
index 48751ce..cf24c8d 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -842,6 +842,7 @@
     skip_T	ctx_skip;
     scope_T	*ctx_scope;	    // current scope, NULL at toplevel
     int		ctx_had_return;	    // last seen statement was "return"
+    int		ctx_had_throw;	    // last seen statement was "throw"
 
     cctx_T	*ctx_outer;	    // outer scope for lambda or nested
 				    // function