Add a check for Custom404

lib/msf/core/exploit/remote/browser_exploit_server.rb

@@ -95,6 +95,14 @@ module Msf
       ], Exploit::Remote::BrowserExploitServer)
     end
 
+    def setup
+      custom_404 = get_custom_404_url
+      if !custom_404.blank? && custom_404 !~ /^http/i
+        raise Msf::OptionValidateError.new(['Custom404 (must begin with http or https)'])
+      end
+      super
+    end
+
     #
     # Returns the custom 404 URL set by the user
     #