GetExternalSigner(): fail if multiple signers are found

If there are multiple external signers, `GetExternalSigner()` will
just pick the first one in the list. If the user has two or more
hardware wallets connected at the same time, he might not notice this.

This PR adds a check and fails with suitable message.

test/functional/rpc_signer.py

@@ -77,10 +77,7 @@ class RPCSignerTest(BitcoinTestFramework):
         )
         self.clear_mock_result(self.nodes[1])
 
-        result = self.nodes[1].enumeratesigners()
-        assert_equal(len(result['signers']), 2)
-        assert_equal(result['signers'][0]["fingerprint"], "00000001")
-        assert_equal(result['signers'][0]["name"], "trezor_t")
+        assert_equal({'fingerprint': '00000001', 'name': 'trezor_t'} in self.nodes[1].enumeratesigners()['signers'], True)
 
 if __name__ == '__main__':
     RPCSignerTest().main()