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_timers.vim b/src/testdir/test_timers.vim
index 963cc23..5b09011 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -1,7 +1,7 @@
 " Test for timers
 
 if !has('timers')
-  finish
+  throw 'Skipped, timers feature missing'
 endif
 
 source shared.vim