[Chronik] Add TokenTxEntry::burn_summary for a nice human-readable burn summary

Summary: SLP/ALP txs can have problems for many reasons, so we add burn_summary, which returns a human-readable string of exactly what went wrong.

Test Plan: cargo test -p bitcoinsuite-slp --lib test_burn_summary

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15103