Don't send InvoiceError on failed authentication

When an invoice or invoice request cannot be authenticated from an
OffersContext, simply do not respond instead of sending an InvoiceError
message. According to BOLT4, messages sent over a blinded path not
intended for its use MUST be ignored.

lightning/src/ln/channelmanager.rs

@@ -10719,10 +10719,7 @@ where
 					},
 					None => match invoice_request.verify_using_metadata(expanded_key, secp_ctx) {
 						Ok(invoice_request) => invoice_request,
-						Err(()) => {
-							let error = Bolt12SemanticError::InvalidMetadata;
-							return responder.respond(OffersMessage::InvoiceError(error.into()));
-						},
+						Err(()) => return ResponseInstruction::NoResponse,
 					},
 				};
 
@@ -10833,7 +10830,7 @@ where
 								})
 						}
 					},
-					Err(()) => Err(InvoiceError::from_string("Unrecognized invoice".to_owned())),
+					Err(()) => return ResponseInstruction::NoResponse,
 				};
 
 				match result {