qt: fix: bip70 pay reqs need x509 verification

regression from https://github.com/spesmilo/electrum/pull/8462

- pr.verify() was called in qml, but not in qt gui
- we now call pr.verify() in get_payment_request(), to make the API less error-prone
- it is now ok to call pr.verify() multiple times, the result is cached

electrum/payment_identifier.py

@@ -340,7 +340,7 @@ class PaymentIdentifier(Logger):
             elif self.bip70:
                 from . import paymentrequest
                 pr = await paymentrequest.get_payment_request(self.bip70)
-                if not pr.error:
+                if pr.verify():
                     self.bip70_data = pr
                     self.set_state(PaymentIdentifierState.MERCHANT_NOTIFY)
                 else: