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.

src/testdir/test_quotestar.vim

@@ -134,6 +134,6 @@ func Test_quotestar()
   let @* = quotestar_saved
 
   if !empty(skipped)
-    throw skipped
+    throw 'Skipped: ' . skipped
   endif
 endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    491,
 /**/
     490,
 /**/