Display fee when paying invoice

[tomsmith8]

Task

If queryRoute is greater than amount + balance the backend will return an error before paying - here

• Display est. fee
• if amount + fee is larger than balance, the backend will return error message "Insufficient balance" upon verifying the invoice.

This will happen when you verify the invoice, we'll get a response from the backend with error message that payment exceeds balance.

Design

[tomastiminskas]

@tomsmith8 I was checking into the relay changes and it's not actually returning the error on queryRoute endpoint, but on payInvoice/ sendKeysend endpoints. So we don't have a way to show fee to user before it confirms the payment.

@Evanfeenstra we will need to apply this to queryRoute endpoint or add a new endpoint to get estimated fee for an invoice or a key send. That way we can show fees to user before he confirms the pmt. Let me know if it makes sense