test: Avoid empty errmsg in JSONRPCException

[kevkevinpal]

mentioned in: bitcoin#34575

"> * I think JSON-RPC 2 is meant to always use HTTP status 200 regardless of error but cannot find it in the specification, I guess adding it is okay, it's being sent in everywhere after all.

See https://github.com/bitcoin/bitcoin/blob/35e6444fdc4068adc79082648f9889ad593e623b/doc/JSON-RPC-interface.md#json-rpc-11-vs-20

• The old output on master

  Method not found (-32601)

  was nicer than the PR's

  {'code': -32601, 'message': 'Method not found'} [http_status=200]

Yeah, happy to review a follow-up improving the wording. One could use something like (untested):

 class JSONRPCException(Exception):
    def __init__(self, rpc_error, http_status=None):
        message = rpc_error.pop("message", "INTERNAL ERROR: message field missing in rpc error dict")
        code = rpc_error.pop("code", "INTERNAL_ERROR: code field missing in rpc error dict")
        extra = f'{rpc_error}' if rpc_error else ''
        super().__init__(f"{message} ({code}) {extra} [http_status={http_status}]")
" - maflcko