patch 8.2.0522: several errors are not tested for

Problem:    Several errors are not tested for.
Solution:   Add tests. (Yegappan Lakshmanan, closes #5892)
diff --git a/src/testdir/test_expand.vim b/src/testdir/test_expand.vim
index 383921b..8606223 100644
--- a/src/testdir/test_expand.vim
+++ b/src/testdir/test_expand.vim
@@ -131,6 +131,7 @@
   call assert_equal(4, winnr('$'))
   call assert_equal('foo!', bufname(winbufnr(1)))
   call assert_equal('foo', bufname(winbufnr(2)))
+  call assert_fails('e %:s/.*//', 'E500:')
   %bwipe!
 endfunc