Add default message if none provided

[justinbeaty]

Resolves #7

Currently if you create an error and don't provide a message, it will be an empty string. This PR makes it so the default message is used from messagesByCodes[].

Examples:

throw new HttpError(401) // message: Unauthorized
throw new UnauthorizedError() // message: Unauthorized
throw new UnauthorizedError({ key1: 'value1' }) // message: Unauthorized

Also added a message !== null check since typeof null === 'object' and it would throw if you called new UnauthorizedError(null). Not sure why someone would do that, but just for extra safety.

[justinbeaty]

@mgcrea is this what you envisioned?