test: restore JSONRPCException error format

This is a follow-up to PR 34575.

Copy is done so that checking of error["code"] in test_node.py
while handling this exception doesn't fail.

Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>

test/functional/test_framework/util.py

@@ -5,6 +5,7 @@
 """Helpful routines for regression testing."""
 
 from base64 import b64encode
+from copy import copy
 from decimal import Decimal
 from subprocess import CalledProcessError
 import hashlib
@@ -30,10 +31,16 @@ logger = logging.getLogger("TestFramework.utils")
 
 class JSONRPCException(Exception):
     def __init__(self, rpc_error, http_status=None):
-        super().__init__(f"{rpc_error} [http_status={http_status}]")
         self.error = rpc_error
         self.http_status = http_status
 
+        # throw KeyError if any required fields are missing
+        copied_error = copy(rpc_error)
+        message = copied_error.pop("message")
+        code = copied_error.pop("code")
+        extra = f'{copied_error}' if copied_error else ''
+        super().__init__(f"{message} ({code}) {extra} [http_status={http_status}]")
+
 # Assert functions
 ##################