patch 8.2.4678: Vim9: not all code is tested

Problem:    Vim9: not all code is tested.
Solution:   Add a few more tests.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index a6789b5..483fe1e 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2636,7 +2636,10 @@
 			SOURCING_LNUM = iptr->isn_lnum;
 			if (do_source(si->sn_name, FALSE, DOSO_NONE, NULL)
 								       == FAIL)
+			{
+			    semsg(_(e_cant_open_file_str_2), si->sn_name);
 			    goto on_error;
+			}
 		    }
 		}
 		break;