Make script_tests.cpp output verbose errors.

Summary:
Currently, when a test from script_test.json has an invalid format
the test runner outputs obtuse error messages. This commit
makes it so the test runner outputs the information that allows
programmers to figure out which JSON test has failed.

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1300