tests: Add exception error message for JSONRPCException #8810

This improves error reporting if JSONRPCException is not specifically caught and ends up in Python's default backtrace handler.

Before:

Traceback (most recent call last):
  File "/.../projects/bitcoin/bitcoin/qa/rpc-tests/test_framework/authproxy.py", line 153, in __call__
    raise JSONRPCException(response['error'])
test_framework.authproxy.JSONRPCException

After:

Traceback (most recent call last):
  File "/.../projects/bitcoin/bitcoin/qa/rpc-tests/test_framework/authproxy.py", line 152, in __call__
    raise JSONRPCException(response['error'])
test_framework.authproxy.JSONRPCException: Unknown named parameter random (-8)

Could make sense to also cherry-pick to upstream?

Yes, would make sense in this case.

Looks like upstream already has similar functionality: https://github.com/jgarzik/python-bitcoinrpc/blob/master/bitcoinrpc/authproxy.py#L58 I like this solution better, though, as it also shows the error code.

You could cherry-pick upstream (https://github.com/jgarzik/python-bitcoinrpc/pull/48)instead and modify it, so that the error code is passed as well...

Though, I don't feel strong about this. Unfortunately we already diverged from upstream quite a bit and it turns out for every problem in this file there are two comparable but different solutions. (One for Bitcoin Core and the other for the lib) So we are already "out of sync".

Yes I don't care deeply about this either. The projects have different goals, our copy is just for testing the version of Bitcoin Core in the repository while the upstream projects tries to keep as much compatibility as possible.