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_gui.vim b/src/testdir/test_gui.vim
index 5a1763d..d030a6a 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -2,7 +2,7 @@
 
 source shared.vim
 if !CanRunGui()
-  finish
+  throw 'Skipped, cannot run GUI'
 endif
 
 source setup_gui.vim