Rename `RedisNIOError` to `RedisClientError`

Motivation:

To make it a little more generic, and to avoid turnover during renames (such as the planned rebranding in issue #61 (closed)), 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.