Using Sintra's error block and Environment Variable

Hi! 

I have a quite large Sinatra application that is compose of multiple Sinatra apps. Moreover, rack is being used to mount each app into a URL endpoint. For example: /admin, /api, /report 

I created this pull request to demostrate how we can catch all the error using the Sinatra error block. Therefore, not mixing error reporting code on our endpoint. I really didn't want to use begin rescue into each action. 


If you feel it is a good idea, I could create another example becuase I subclass Sinatra::Base and did something like: 

```ruby
class MealsSinatraBase < Sinatra::Base

  configure do
    Rollbar.configure do |config|
      config.access_token = ''
      config.environment = Sinatra::Base.environment
      config.root = Dir.pwd
    end
  end

  error do
    Rollbar.report_exception(env['sinatra.error'])
    "error"
  end
end

class AdminApp < MealsSinatraBase
end

class ReportApp < MealsSinatraBase
end

class API < MealsSinatraBase
end
```

I hope this help other Sinatra heads. 


:thumbsup: