I can't leave a comment because I have no reputation, but I really want to share this with you:

https://github.com/sferik/twitter/blob/master/lib/twitter/error.rb

I think this is definitely one of the good/best ways of organizationing your exceptions.

A realy good way I've seen for organizing exceptions in a Ruby library is in sferik's twitter library.

https://github.com/sferik/twitter/blob/master/lib/twitter/error.rb

By using Ruby's dynamic class creation mechanism with Class.new(ParentClass) It makes it very easy to reason about the class hierarchy.

Client related exceptions inherit from ClientError. Server related exceptions inherit from ServerError. ClientError and ServerError both inherit from Twitter::Error

HTTP response codes are mapped to error classes and raised upon receiving the HTTP response:

@parser = Http::Parser.new(http_response)
error = Twitter::Error::ERRORS[@parser.status_code]
raise error if error

I've abreviated the code to only show the important parts:

module Twitter
  class Error < StandardError
    ...    
    ClientError = Class.new(self)
    BadRequest = Class.new(ClientError)
    Unauthorized = Class.new(ClientError)
    RequestEntityTooLarge = Class.new(ClientError)
    NotFound = Class.new(ClientError)
    NotAcceptable = Class.new(ClientError)
    UnprocessableEntity = Class.new(ClientError)
    TooManyRequests = Class.new(ClientError)
    
    Forbidden = Class.new(ClientError)
    AlreadyFavorited = Class.new(Forbidden)
    AlreadyRetweeted = Class.new(Forbidden)
    DuplicateStatus = Class.new(Forbidden)

    
    ServerError = Class.new(self)
    InternalServerError = Class.new(ServerError)
    BadGateway = Class.new(ServerError)
    ServiceUnavailable = Class.new(ServerError)
    GatewayTimeout = Class.new(ServerError)

    ERRORS = {
      400 => Twitter::Error::BadRequest,
      401 => Twitter::Error::Unauthorized,
      403 => Twitter::Error::Forbidden,
      404 => Twitter::Error::NotFound,
      406 => Twitter::Error::NotAcceptable,
      413 => Twitter::Error::RequestEntityTooLarge,
      422 => Twitter::Error::UnprocessableEntity,
      429 => Twitter::Error::TooManyRequests,
      500 => Twitter::Error::InternalServerError,
      502 => Twitter::Error::BadGateway,
      503 => Twitter::Error::ServiceUnavailable,
      504 => Twitter::Error::GatewayTimeout,
    }.freeze
    ...
  end
end