Use custom Error subclasses instead of instancing Error directly

[Acksell]

It would be nice if you could identify an error by its class rather than the message that is thrown. This allows the client to more easily adhere to the principle of only catching errors that you intend to catch.

Specifically in lib/decode.js.

[patrickhulce]

Good idea! Wanna give this a shot? The below might be a decent start

class JPEGError extends Error {
	constructor(message) {
		super(message);
		this.name = 'JPEGError';
	}
}