Motivation:

To make it a little more generic, and to avoid turnover during renames (such as the planned rebranding in issue #61), `RedisClientError` more accurately reflects the source of the errors, as well as the responsibility of causing the bug.

Modifications:

- Rename `RedisNIOError` to `RedisClientError`
- Rename `RedisError` file to `RedisErrors`
- Add documentation of `RedisClientError`
- Remove no longer used `.unsupportedOperation(method:message:)` value
- Rename `.responseConversion(to:)` to `.failedRESPConversion(to:)`

Result:

Names of `RedisClientError` should be more descriptive, less prone to turnover, and more documented for users to understand the issues related to these thrown errors.