patch 8.2.3345: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add a few more tests. (Dominique Pellé, closes #8757)

src/testdir/test_arglist.vim

@@ -82,6 +82,10 @@ func Test_argadd()
   new
   arga
   call assert_equal(0, len(argv()))
+
+  if has('unix')
+    call assert_fails('argadd `Xdoes_not_exist`', 'E479:')
+  endif
 endfunc
 
 func Test_argadd_empty_curbuf()
@@ -434,6 +438,8 @@ func Test_argdelete()
   argdel
   call Assert_argc(['a', 'c', 'd'])
   %argdel
+
+  call assert_fails('argdel does_not_exist', 'E480:')
 endfunc
 
 func Test_argdelete_completion()