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/wallet_signer.py

@@ -32,6 +32,13 @@ class WalletSignerTest(BitcoinTestFramework):
         else:
             return path
 
+    def mock_multi_signers_path(self):
+        path = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'mocks', 'multi_signers.py')
+        if platform.system() == "Windows":
+            return "py " + path
+        else:
+            return path
+
     def set_test_params(self):
         self.num_nodes = 2
         # The experimental syscall sandbox feature (-sandbox) is not compatible with -signer (which
@@ -58,6 +65,8 @@ class WalletSignerTest(BitcoinTestFramework):
         self.test_valid_signer()
         self.restart_node(1, [f"-signer={self.mock_invalid_signer_path()}", "-keypool=10"])
         self.test_invalid_signer()
+        self.restart_node(1, [f"-signer={self.mock_multi_signers_path()}", "-keypool=10"])
+        self.test_multiple_signers()
 
     def test_valid_signer(self):
         self.log.debug(f"-signer={self.mock_signer_path()}")
@@ -212,5 +221,11 @@ class WalletSignerTest(BitcoinTestFramework):
         self.log.info('Test invalid external signer')
         assert_raises_rpc_error(-1, "Invalid descriptor", self.nodes[1].createwallet, wallet_name='hww_invalid', disable_private_keys=True, descriptors=True, external_signer=True)
 
+    def test_multiple_signers(self):
+        self.log.debug(f"-signer={self.mock_multi_signers_path()}")
+        self.log.info('Test multiple external signers')
+
+        assert_raises_rpc_error(-1, "GetExternalSigner: More than one external signer found", self.nodes[1].createwallet, wallet_name='multi_hww', disable_private_keys=True, descriptors=True, external_signer=True)
+
 if __name__ == '__main__':
     WalletSignerTest().main()