patch 8.2.3783: confusing error for using a variable as a function

Problem:    Confusing error for using a variable as a function.
Solution:   If a function is not found but there is a variable, give a more
            useful error. (issue #9310)

src/structs.h

@@ -2001,6 +2001,8 @@ typedef struct {
     dict_T	*selfdict;	// Dictionary for "self"
     typval_T	*basetv;	// base for base->method()
     type_T	*check_type;	// type from funcref or NULL
+    int		fe_found_var;	// if the function is not found then give an
+				// error that a variable is not callable.
 } funcexe_T;
 
 /*