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_textprop.vim b/src/testdir/test_textprop.vim
index bfd39d5..cb27f58 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -2,7 +2,7 @@
 " buffer.
 
 if !has('textprop')
-  finish
+  throw 'Skipped, textprop feature missing'
 endif
 
 source screendump.vim