Make sure when the backend cannot be downloaded or the site resolved that we're forgiving until we try to use the provided FFI functions.

main.rkt

@@ -1,5 +1,6 @@
 #lang racket/base
 
+(require "private/racket-webview-downloader.rkt")
 (require "private/racket-webview.rkt")
 (require "private/wv-context.rkt")
 (require "private/wv-window.rkt")
@@ -18,6 +19,7 @@
                        "private/rgba.rkt"
                        "private/mimetypes.rkt"
                        "private/menu.rkt"
+                       "private/racket-webview-downloader.rkt"
                        )
          webview-set-loglevel
          webview-version