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_tcl.vim b/src/testdir/test_tcl.vim
index c0eadc6..8ccd34c 100644
--- a/src/testdir/test_tcl.vim
+++ b/src/testdir/test_tcl.vim
@@ -1,7 +1,7 @@
 " Tests for the Tcl interface.
 
 if !has('tcl')
-  finish
+  throw 'Skipped, tcl feature missing'
 end
 
 " Helper function as there is no builtin tcleval() function similar