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_signs.vim b/src/testdir/test_signs.vim
index 8a6731c..4a23e4b 100644
--- a/src/testdir/test_signs.vim
+++ b/src/testdir/test_signs.vim
@@ -1,7 +1,7 @@
 " Test for signs
 
 if !has('signs')
-  finish
+  throw 'Skipped, signs feature missing'
 endif
 
 func Test_sign()