Catch already paid invoices when sending

[gudnuf]

If an invoice has already been paid by the mint it will return the following error

{"code":20006,"error":"Invoice already paid.","detail":null}

We should catch this when the melt quote is being fetched.

[orveth]

Closing — already-paid invoice errors get toasted now (PR #1027). Followup note: the message 'Melt quote already paid or pending.' isn't user-friendly. Worth considering custom domain error strings for mint operation errors as a separate cleanup.

[gudnuf]

already-paid invoice errors get toasted now (PR #1027).

This PR was for spark, but the mint returns a MintOperationError and we toast that in the send input. Currently, it retries once, but should no retry when #1090 lands