fix panic on malformed discovery requests

[danielwolfman]

Summary

• return an HTTP error response when discovery receives a malformed addUser request
• stop unwrapping discovery handler errors in the HTTP service path
• add an unreleased changelog entry for the discovery crash fix

Context

• observed downstream in Crash on connect Spotifyd/spotifyd#1352 with Creating SHA1 HMAC failed for base key []
• malformed discovery requests should not crash the receiver process

Validation

• cargo fmt --all
• cargo build -j 2
• cargo clippy -j 2