Podcaster: verify existence of URL link.

[Ignigena]

For the new referencing feature a user can specify the URL of their site to link a node to a specific Podcaster media item.

Need to verify that the URL is both accurate and that the Bethel module has been installed on the remote site in order to ensure errors don't present themselves to the user. The field on the media edit page should both disable itself and tell the user to add the URL or install the module in order to support linking between their website and the media item.