Split assertion in test_node to provide more accurate error messages

Summary: Discovered while working with RPC code.

Test Plan: test_runner.py, while manually triggering these assertions

Reviewers: deadalnix, Mengerian, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1737