Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
@dstillman, idk if we've ever really encountered this before, but should we really `throw` here? I'm thinking more about translators. It doesn't seem like a critical fault, so we could just ignore the attachment and log the error (and send it to zotero.org maybe as well)