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_vartabs.vim b/src/testdir/test_vartabs.vim
index df61c0e..640aa0b 100644
--- a/src/testdir/test_vartabs.vim
+++ b/src/testdir/test_vartabs.vim
@@ -1,7 +1,7 @@
 " Test for variable tabstops
 
 if !has("vartabs")
-  finish
+  throw 'Skipped, vartabs feature missing'
 endif
 
 source view_util.vim