patch 8.2.0056: execution stack is incomplete and inefficient

Problem:    Execution stack is incomplete and inefficient.
Solution:   Introduce a proper execution stack and use it instead of
            sourcing_name/sourcing_lnum.  Create a string only when used.
diff --git a/src/ex_eval.c b/src/ex_eval.c
index 4d76572..382e99e 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -534,15 +534,16 @@
 	goto nomem;
 
     excp->type = type;
-    excp->throw_name = vim_strsave(sourcing_name == NULL
-					      ? (char_u *)"" : sourcing_name);
+    excp->throw_name = estack_sfile();
+    if (excp->throw_name == NULL)
+	excp->throw_name = vim_strsave((char_u *)"");
     if (excp->throw_name == NULL)
     {
 	if (should_free)
 	    vim_free(excp->value);
 	goto nomem;
     }
-    excp->throw_lnum = sourcing_lnum;
+    excp->throw_lnum = SOURCING_LNUM;
 
     if (p_verbose >= 13 || debug_break_level > 0)
     {