Read-Only: Misc Improvements

[kilrau]

• [x] Adjust the text a bit, it should be titled "read-only" etc, I'll do this.
• [x] Warning about custody and "seed" shouldn't be there when I go through the read-only import flow, Title should be "Import L-BTC Read-Only Wallet"
• [x] Should say some more details what's wrong with the descriptor (in my case it was bitcoin instead of liquid)