refactor: use util::Result for GetExternalSigner()

This commit does not change behavior, except that the error message no longer contains the function name.

test/functional/wallet_signer.py

@@ -243,7 +243,7 @@ class WalletSignerTest(BitcoinTestFramework):
         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, external_signer=True)
+        assert_raises_rpc_error(-1, "More than one external signer found", self.nodes[1].createwallet, wallet_name='multi_hww', disable_private_keys=True, external_signer=True)
 
 if __name__ == '__main__':
     WalletSignerTest(__file__).main()