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_digraph.vim b/src/testdir/test_digraph.vim
index 8d56a1a..92d4e47 100644
--- a/src/testdir/test_digraph.vim
+++ b/src/testdir/test_digraph.vim
@@ -1,7 +1,7 @@
 " Tests for digraphs
 
 if !has("digraphs")
-  finish
+  throw 'Skipped, digraphs feature missing'
 endif
 
 func Put_Dig(chars)