Better errors for WebLN prompts

[wbobeirne]

Closes #227

Description

Changes prompt errors from message.error which would disappear automatically, to rendering an <Alert /> that the user has to manually dismiss. Doesn't look quite as nice, but should be a lot better for longer errors or when the user backgrounds the window and comes back later.

Steps to Test

1. Go to https://webln.dev/#/api/make-invoice
2. Make an invoice of a number way too high
3. Confirm you get an error
4. Confirm the error doesn't disappear automatically
5. Confirm clicking on the X dismisses the error

Screenshots