Test framework JSONRPCException should specify which node failed #12945

I can guess from context, and by trial and error, which node was throwing this exception (a v0.15 node that I tested for backwards compatibility while rebasing #12134). But it's not very clear from the messages.

 ... 
 node0 2018-04-11T13:35:49.811352Z Received a POST request for / from 127.0.0.1:49312 
 node0 2018-04-11T13:35:49.811429Z ThreadRPCServer method=syncwithvalidationinterfacequeue user=__cookie__ 
 node1 2018-04-11T13:35:49.811966Z Received a POST request for / from 127.0.0.1:49314 
 node1 2018-04-11T13:35:49.812043Z ThreadRPCServer method=syncwithvalidationinterfacequeue user=__cookie__ 
 test  2018-04-11T13:35:49.813000Z TestFramework (ERROR): JSONRPC error

Traceback (most recent call last):

  File "/Volumes/SSD/src/bitcoin/test/functional/test_framework/test_framework.py", line 146, in main

    self.setup_network()

  File "/Volumes/SSD/src/bitcoin/test/functional/test_framework/test_framework.py", line 224, in setup_network

    self.sync_all()

  File "/Volumes/SSD/src/bitcoin/test/functional/test_framework/test_framework.py", line 331, in sync_all

    sync_mempools(group)

  File "/Volumes/SSD/src/bitcoin/test/functional/test_framework/util.py", line 394, in sync_mempools

    r.syncwithvalidationinterfacequeue()

  File "/Volumes/SSD/src/bitcoin/test/functional/test_framework/coverage.py", line 47, in __call__

    return_val = self.auth_service_proxy_instance.__call__(*args, **kwargs)

  File "/Volumes/SSD/src/bitcoin/test/functional/test_framework/authproxy.py", line 138, in __call__

    raise JSONRPCException(response['error'])

test_framework.authproxy.JSONRPCException: Method not found (-32601) 
 test  2018-04-11T13:35:49.814000Z TestFramework (INFO): Stopping nodes 
 test  2018-04-11T13:35:49.814000Z TestFramework.node0 (DEBUG): Stopping node 
 node0 2018-04-11T13:35:49.814963Z Received a POST request for / from 127.0.0.1:49312 
...

cc @jnewbery

At the moment, the AuthServiceProxy object doesn't know anything about which node it's connected to. You could easily change that by updating get_rpc_proxy() to pass in the node_number, have that stored by AuthServiceProxy's __init__ method, and then have it passed into the JSONRPCException.

It's quite unusual that the stack trace doesn't indicate which node the RPC is being called on, and it's fairly easy to add pdb tracepoints or additional logging where that's not the case, so I don't know how useful this would be in general.

Maybe close as up for grabs?