tests: Add exception error message for JSONRPCException
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.JSONRPCExceptionAfter:
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)