Add extra info to script parsing error to make debugging easier

Summary: Parsing errors in /src/test/data/*.json files give unknown location(0): fatal error in "script_json_test": std::runtime_error: script parse error which is totally useless for debugging. Related to T312.

Test Plan: make check + make check with a line that doesn't parse (error message now prints the line's contents, making debugging easier)

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1289