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_pyx2.vim b/src/testdir/test_pyx2.vim
index baaf7fa..64bd579 100644
--- a/src/testdir/test_pyx2.vim
+++ b/src/testdir/test_pyx2.vim
@@ -2,7 +2,7 @@
 
 set pyx=2
 if !has('python')
-  finish
+  throw 'Skipped, python feature missing'
 endif
 
 let s:py2pattern = '^2\.[0-7]\.\d\+'