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_prompt_buffer.vim b/src/testdir/test_prompt_buffer.vim
index 1b8a1ec..5cf65d5 100644
--- a/src/testdir/test_prompt_buffer.vim
+++ b/src/testdir/test_prompt_buffer.vim
@@ -1,7 +1,7 @@
 " Tests for setting 'buftype' to "prompt"
 
 if !has('channel')
-  finish
+  throw 'Skipped, channel feature missing'
 endif
 
 source shared.vim