Validations on required fields and data types

[salimhb]

The Pin structure is defined by the Pebble API documentation
Missing required fields should be detected by the gem. Field types are also defined by the documentation and should be validated by the gem.

The API doesn't isn't very strict and still in some cases returns a success response when required fields are missing or invalid data types are sent for certain fields. Eventhough the API response is success, the actual PIN is not pushed to the watch, which is confusing.