Mint: LNbitsWallet add extra check for payment state

[callebtc]

Needs testing:

• mint with lnbits backend with lnd backend
• create melt quote
• turn off lnd
• try melting
• melt should fail

[minibits-cash]

PR tested using test mint. My test mint does not use LND but LNbits with Fakewallet.

1. Test with melt while LNbits running - OK
2. Test with melt while LNbits turned off - mint returned error:

return PaymentResponse(error_message=r.json()["detail"])
UnboundLocalError: local variable 'r' referenced before assignment"

When looking at the code "r" seems to be defined only inside of try block, so not available in catch block to return proper error message?