patch 8.1.0875: not all errors of marks and findfile()/finddir() are tested

Problem:    Not all errors of marks and findfile()/finddir() are tested.
Solution:   Add more test coverage. (Dominique Pelle)

src/testdir/test_marks.vim

@@ -133,3 +133,44 @@ func Test_marks_cmd_multibyte()
 
   bwipe!
 endfunc
+
+func Test_delmarks()
+  new
+  norm mx
+  norm `x
+  delmarks x
+  call assert_fails('norm `x', 'E20:')
+
+  " Deleting an already deleted mark should not fail.
+  delmarks x
+
+  " Test deleting a range of marks.
+  norm ma
+  norm mb
+  norm mc
+  norm mz
+  delmarks b-z
+  norm `a
+  call assert_fails('norm `b', 'E20:')
+  call assert_fails('norm `c', 'E20:')
+  call assert_fails('norm `z', 'E20:')
+  call assert_fails('delmarks z-b', 'E475:')
+
+  call assert_fails('delmarks', 'E471:')
+  call assert_fails('delmarks /', 'E475:')
+
+  " Test delmarks!
+  norm mx
+  norm `x
+  delmarks!
+  call assert_fails('norm `x', 'E20:')
+  call assert_fails('delmarks! x', 'E474:')
+
+  bwipe!
+endfunc
+
+func Test_mark_error()
+  call assert_fails('mark', 'E471:')
+  call assert_fails('mark xx', 'E488:')
+  call assert_fails('mark _', 'E191:')
+endfunc