patch 8.2.3720: Vim9: Internal error when invoking closure in legacy context

Problem:    Vim9: Internal error when invoking closure in legacy context.
Solution:   Give a more appropriate error message. (closes #9251)

src/errors.h

@@ -689,3 +689,5 @@ EXTERN char e_cannot_find_variable_to_unlock_str[]
 	INIT(= N_("E1246: Cannot find variable to (un)lock: %s"));
 EXTERN char e_line_number_out_of_range[]
 	INIT(= N_("E1247: Line number out of range"));
+EXTERN char e_closure_called_from_invalid_context[]
+	INIT(= N_("E1248: Closure called from invalid context"));

src/testdir/test_vim9_func.vim

@@ -2384,6 +2384,21 @@ def Test_global_closure_called_directly()
   delfunc g:Inner
 enddef
 
+def Test_closure_called_from_legacy()
+  var lines =<< trim END
+      vim9script
+      def Func()
+        var outer = 'foo'
+        var F = () => {
+              outer = 'bar'
+            }
+        execute printf('call %s()', string(F))
+      enddef
+      Func()
+  END
+  CheckScriptFailure(lines, 'E1248')
+enddef
+
 def Test_failure_in_called_function()
   # this was using the frame index as the return value
   var lines =<< trim END

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3720,
 /**/
     3719,
 /**/

src/vim9execute.c

@@ -2733,7 +2733,13 @@ exec_instructions(ectx_T *ectx)
 		    if (outer == NULL)
 		    {
 			SOURCING_LNUM = iptr->isn_lnum;
-			iemsg("LOADOUTER depth more than scope levels");
+			if (ectx->ec_frame_idx == ectx->ec_initial_frame_idx
+						 || ectx->ec_outer_ref == NULL)
+			    // Possibly :def function called from legacy
+			    // context.
+			    emsg(_(e_closure_called_from_invalid_context));
+			else
+			    iemsg("LOADOUTER depth more than scope levels");
 			goto theend;
 		    }
 		    tv = ((typval_T *)outer->out_stack->ga_data)