Remove error namespace

[annihilatorq]

In the initial design, I based the implementation on the error namespace because that was the established practice in asio. Looking at the library now, there is a conceptual difference between aero and asio: asio uses a regular enum, which causes its errors to propagate to namespace error, making error handling as pleasant and familiar as possible: asio::error::not_connected, whereas aero uses an enum class. I suspect that asio uses regular enums because enum classes were introduced in C++11, and asio needs to support earlier standards. In the end, it seems to me that, to maintain a consistent API, the decision was made not to switch to enum classes at all after C++11 (which isn’t so bad, but I think it just complicates the naming of error codes).

What I'm getting at is this: this PR eliminates the need for users to write something like websocket::error::handshake_error::status_code_invalid,
allowing them to shorten the code to
websocket::handshake_error::status_code_invalid.
And while this is still isn't short, it's clearly the best compromise between readability and simplicity at this point.