Give more descriptive error when parsing partial-byte hex values

Summary: Fixes T312. 0x0 and 0x001 are examples of hex values that fail the script parser. This isn't immediately obvious unless you're familiar with the code, and the script fails with a totally useless and generic error message.

Test Plan: make check + make check with script_tests.json having one line changed 0x02 -> 0x2 (fails with the expected error)

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T312

Differential Revision: https://reviews.bitcoinabc.org/D1290