[Chronik] Add should_ignore to empp::ParseError

Summary:
When reporting errors from the indexer, it isn't helpful to know that e.g. a P2PKH-only tx isn't eMPP compliant and Chronik shouldn't refuse to broadcast them.

However, if a script starts with OP_RETURN OP_RESERVED, it's pretty clear that the intent is to use eMPP, and therefore we report the error and refuse broadcast.

Also, refactor some small things like using Opcode instead of Op in ParseError.

Test Plan: cargo test -p bitcoinsuite-slp

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14999