patch 8.0.0491: quotestar test fails when features are missing

Problem:    The quotestar test fails when a required feature is missing.
Solution:   Prepend "Skipped" to the thrown exception.
diff --git a/src/testdir/test_quotestar.vim b/src/testdir/test_quotestar.vim
index 300a964..111a226 100644
--- a/src/testdir/test_quotestar.vim
+++ b/src/testdir/test_quotestar.vim
@@ -134,6 +134,6 @@
   let @* = quotestar_saved
 
   if !empty(skipped)
-    throw skipped
+    throw 'Skipped: ' . skipped
   endif
 endfunc