wallet: return and display signer error

Both RPC and GUI now render a useful error message instead of (silently) failing.

Replace bool with util::Result<void> to clarify that this either succeeds or returns an error message.

test/functional/mocks/signer.py

@@ -49,6 +49,7 @@ def displayaddress(args):
         "sh(wpkh([00000001/49h/1h/0h/0/0]02c97dc3f4420402e01a113984311bf4a1b8de376cac0bdcfaf1b3ac81f13433c7))#kz9y5w82": "2N2gQKzjUe47gM8p1JZxaAkTcoHPXV6YyVp",
         "pkh([00000001/44h/1h/0h/0/0]02c97dc3f4420402e01a113984311bf4a1b8de376cac0bdcfaf1b3ac81f13433c7)#q3pqd8wh": "n1LKejAadN6hg2FrBXoU1KrwX4uK16mco9",
         "tr([00000001/86h/1h/0h/0/0]c97dc3f4420402e01a113984311bf4a1b8de376cac0bdcfaf1b3ac81f13433c7)#puqqa90m": "tb1phw4cgpt6cd30kz9k4wkpwm872cdvhss29jga2xpmftelhqll62mscq0k4g",
+        "wpkh([00000001/84h/1h/0h/0/1]03a20a46308be0b8ded6dff0a22b10b4245c587ccf23f3b4a303885be3a524f172)#aqpjv5xr": "wrong_address",
     }
     if args.desc not in expected_desc:
         return sys.stdout.write(json.dumps({"error": "Unexpected descriptor", "desc": args.desc}))

test/functional/wallet_signer.py

@@ -141,6 +141,13 @@ class WalletSignerTest(BitcoinTestFramework):
         )
         self.clear_mock_result(self.nodes[1])
 
+        # Returned address MUST match:
+        address_fail = hww.getnewaddress(address_type="bech32")
+        assert_equal(address_fail, "bcrt1ql7zg7ukh3dwr25ex2zn9jse926f27xy2jz58tm")
+        assert_raises_rpc_error(-1, 'Signer echoed unexpected address wrong_address',
+            hww.walletdisplayaddress, address_fail
+        )
+
         self.log.info('Prepare mock PSBT')
         self.nodes[0].sendtoaddress(address4, 1)
         self.generate(self.nodes[0], 1)