Handle HTTP Data API errors

[pilaf]

Currently we only handle 500 errors with FileMaker server error codes.

The Data API has a set of standard HTTP 4xx errors that it can respond with that we're currently not accounting for, but we should. These should probably raise exceptions with their own namespaces, maybe FmRest::HTTPError (as opposed to FmRest::APIError). More discussion may be needed for settling on the right naming scheme though.

See: https://fmhelp.filemaker.com/docs/17/en/dataapi/#error-responses