catch sxref failures and report as warnings when loading help data

svn: r8063

original commit: b78f8acb504cf8e6100e754baf58c19b9c989dd4

collects/scribble/xref.ss

@@ -40,7 +40,9 @@
          [ci (send renderer collect null null)])
     (for-each (lambda (src)
                 (parameterize ([current-namespace (namespace-anchor->empty-namespace here)])
-                  (send renderer deserialize-info (src) ci)))
+                  (let ([v (src)])
+                    (when v
+                      (send renderer deserialize-info v ci)))))
               sources)
     (make-xrefs renderer (send renderer resolve null null ci))))
 

collects/scribblings/scribble/xref.scrbl

@@ -27,7 +27,8 @@ by @scheme[load-xref], @scheme[#f] otherwise.}
 
 Creates a cross-reference record given a list of functions that each
 produce a serialized information obtained from @xmethod[render%
-serialize-info].
+serialize-info]. If a @scheme[sources] element produces @scheme[#f],
+its result is ignored.
 
 Since the format of serialized information is specific to a rendering
 class, the optional @scheme[using-render%] argument accepts the