patch 8.1.1524: tests are silently skipped

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.
diff --git a/src/testdir/test_matchadd_conceal_utf8.vim b/src/testdir/test_matchadd_conceal_utf8.vim
index d280c15..fb27577 100644
--- a/src/testdir/test_matchadd_conceal_utf8.vim
+++ b/src/testdir/test_matchadd_conceal_utf8.vim
@@ -1,6 +1,7 @@
 " Test for matchadd() and conceal feature using utf-8.
+
 if !has('conceal')
-  finish
+  throw 'Skipped, conceal feature missing'
 endif
 
 if !has('gui_running') && has('unix')