crazystupid_dsl.rb

module MyApp
    class APIError < Exception; end
    
    class Web < Sinatra::Base
        def self.api(&block)
            # some crazy stuff to wrap any get/put/delete/post/patch
            # requests into a block with automatic error handling, and
            # pass on all other requests to the sinatra base class.
        end

        api do
            get '/hi' do
                raise APIError, "Not implemented!"
            end

            put '/hi' do
                raise APIError, "Not implemented!"
            end
        end
    end
end

internal_error_handling.rb

module MyApp
    class APIError < Exception; end

    class Web < Sinatra::Base
        get '/hi' do
            raise APIError, "Not implemented!"
        rescue APIError => e
            json :ok => false, :message => e.to_s
        end
    end
end

std_error_handling.rb

module MyApp
    class APIError < Exception; end

    class Web < Sinatra::Base
        error APIError do
            json :ok => false, :message => env['whatever the exception key is'].to_s
        end
        
        get '/hi' do
            raise APIError, "Not implemented!"
        end
    end
end