Feature request: validate "Application" with existing swaps

Please see https://lightningnetwork.plus/swaps/8289#comment_55205

I have, boneheadedly, applied to a swap without checking the node immediately prior to my chain on whether or not an existing connection already exists. It would be fantastic, as the network grows larger, a warning of some sort can be provided so future LNers won't make the same mistake as I did.

Thank you!