feat: Add custom UnauthorizedException + mapper

Resolves #52 (closed)

This MR adds a custom UnauthorizedException as well as a corresponding mapper class. This approach was used over the javax.rs.NotAuthorizedException since it exhibited odd behavior when paired with a mapper.

Logging the javax.rs.NotAuthorizedException within the ExceptionMapper would not reveal the cause. This behavior is not desirable since it does not provide adequate information when it is thrown in response to a separate exception bubbling up.