Active Resource should warn about missing prefix value

require 'active_resource'

class Comment < ActiveResource::Base
  self.site = "http://someip.com/posts/:post_id/"
end

Comment.find(1)

#=> ActiveResource::ResourceNotFound: Failed.  Response code = 404.  Response message = Not Found.

This exception is not specific enough because here the first problem is that we didn't pass the :post_id param, see:

Comment.element_path(1)
# => # => /posts//comments/1.xml

and sometimes this situation could be confusing, the resource will never be found if you don't provide the prefix_params, so prefix_params should be a required param IMO.

I attached a patch that adds a new exception ActiveResource::MissingPrefixParam and works like this:

class Comment < ActiveResource::Base
  self.site = "http://someip.com/posts/:post_id/"
end

Comment.find(1)
# => ActiveResource::MissingPrefixParam: post_id prefix_option is missing

Comment.element_path(1)
# => ActiveResource::MissingPrefixParam: post_id prefix_option is missing