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/mocks/multi_signers.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+# Copyright (c) 2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+import argparse
+import json
+import sys
+
+def enumerate(args):
+    sys.stdout.write(json.dumps([{"fingerprint": "00000001", "type": "trezor", "model": "trezor_t"},
+        {"fingerprint": "00000002", "type": "trezor", "model": "trezor_one"}]))
+
+parser = argparse.ArgumentParser(prog='./multi_signers.py', description='External multi-signer mock')
+
+subparsers = parser.add_subparsers(description='Commands', dest='command')
+subparsers.required = True
+
+parser_enumerate = subparsers.add_parser('enumerate', help='list available signers')
+parser_enumerate.set_defaults(func=enumerate)
+
+
+if not sys.stdin.isatty():
+    buffer = sys.stdin.read()
+    if buffer and buffer.rstrip() != "":
+        sys.argv.extend(buffer.rstrip().split(" "))
+
+args = parser.parse_args()
+
+args.func(args)