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rust: Fix "panic message is not a string literal" warnings

Authored by SimonSapin.

Description

rust: Fix "panic message is not a string literal" warnings

These deprecation warnings would not become errors until we actively port
crates to the (not yet released) Rust 2021 edition, but fixing them anyway
reduces console output noise.

Differential Revision: https://phab.mercurial-scm.org/D10743