HTTP response errors

The Merchant API uses the following error codes:

Error code	Description
`400`	Bad Request -- Your request is invalid. Ensure that all the parameters provided in the request follow the API Reference.
`401`	Unauthorized -- Your access token is wrong.
`403`	Forbidden -- Access to the requested resource or action is forbidden.
`404`	Not Found -- The requested resource could not be found.
`405`	Method Not Allowed -- You tried to access an endpoint with an invalid method.
`406`	Not Acceptable -- You requested a format that isn't JSON.
`422`	Unprocessable entity -- Your request is incorrect. This can happen when you're trying to perform an operation that is invalid. For example, trying to use the Capture an order endpoint on an order that is not in `authorised` state.
`429`	Too Many Requests -- You're sending too many requests.
`500`	Internal Server Error -- We had a problem with our server. Try again later.
`503`	Service Unavailable -- We're temporarily offline for maintenance. Please try again later.