Older versions of LND 404 on message signatures

[Will O'Beirne]

Description of the Issue

Because of this issue: lightningnetwork/lnd#2096 older versions of LND are missing sign message and verify message REST endpoints. The current behavior just shows a quick error that says "Not found". It would be much better if we informed the user that their version doesn't support it in Joule, and CLI instructions on how to perform the signature / verification.

Steps to Reproduce

• Use LND 0.5.0
• Sign a message with Joule (https://webln.dev/#/api/sign-message)
• Confirm you get the error

[Will O'Beirne]

I'm going to close this as we're now just going to warn people about sub 0.7.1 versions, so everyone should upgrade anyway.