implement std::error::Error for RedisError and cleanup

[JonasKruckenberg]

No description provided.

[Copilot]

Pull request overview

This PR updates the crate’s internal error handling by making RedisError implement std::error::Error, centralizing string rendering via a new to_str() helper, and introducing typed variants for specific conversion failures (NUL / UTF-8).

Changes:

• Implement fmt::Display + std::error::Error for RedisError, and add RedisError::to_str() for consistent error formatting.
• Add new RedisError variants for NulError and Utf8Error, and wire them into existing call sites (e.g., hash_get_multi).
• Simplify Context::reply error handling to a single Err(err) branch using to_str().

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/rediserror.rs	Adds Error impl, to_str(), and new error variants; changes conversion behavior.
src/raw.rs	Maps CString::new NUL failures into a new RedisError variant.
src/context/mod.rs	Simplifies replying with errors by formatting all RedisErrors via to_str().

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[kei-nan]

Asked copilot to review
@JonasKruckenberg can you take a look at the comments it left?