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_netbeans.vim b/src/testdir/test_netbeans.vim
index 66177ad..836bddf 100644
--- a/src/testdir/test_netbeans.vim
+++ b/src/testdir/test_netbeans.vim
@@ -1,15 +1,14 @@
 " Test the netbeans interface.
 
 if !has('netbeans_intg')
-  finish
+  throw 'Skipped, netbeans_intg feature missing'
 endif
 
 source shared.vim
 
 let s:python = PythonProg()
 if s:python == ''
-  " Can't run this test.
-  finish
+  throw 'Skipped, python program missing'
 endif
 
 " Run "testfunc" after sarting the server and stop the server afterwards.