Add `Equatable` conformance for `RedisError`

Nathan Harrisrequested to merge
rediserror-equatable into master

Motivation:

There is a reasonable way to compare if two RedisErrors are equal, which was seen as needed in the Equatable conformance for RESPValue.

Modifications:

Added Equatable conformance for RedisError by comparing the messages.

Result:

Two RedisError instances are now equatable.

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