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_python3.vim b/src/testdir/test_python3.vim
index aacc5a5..e001360 100644
--- a/src/testdir/test_python3.vim
+++ b/src/testdir/test_python3.vim
@@ -2,7 +2,7 @@
 " TODO: move tests from test88.in here.
 
 if !has('python3')
-  finish
+  throw 'Skipped, python3 feature missing'
 endif
 
 func Test_py3do()