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_makeencoding.vim b/src/testdir/test_makeencoding.vim
index d18b3b6..6de3c70 100644
--- a/src/testdir/test_makeencoding.vim
+++ b/src/testdir/test_makeencoding.vim
@@ -4,8 +4,7 @@
 
 let s:python = PythonProg()
 if s:python == ''
-  " Can't run this test.
-  finish
+  throw 'Skipped, python program missing'
 endif
 
 let s:script = 'test_makeencoding.py'