Improve TestNodeCLI output parsing

Summary:
Parse JSONRPCException errors, and avoid JSON decode exception if RPC
method returns a plain string.

Partial backport of core PR11970 (commit fcfb952)

Depends on D2395

Test Plan:

./test/functional/test_runner.py interface_bitcoin_cli

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2396