Throw better exceptions for `api_usage_exceeded` error

[yoannisj]

The coconut API returns a 403 HTTP status code when the usage limit is exceeded. This throws a genericCoconut\Error, which does not provide much information to the php code using this library.

Ideally, the error message and error code would be part of the exception thrown so it can be displayed to the end user, or the exception can be handled differently (for example: retrying the API call after a delay?).

[yoannisj]

The following line should probably check for http status codes between 400 and 403:

https://github.com/opencoconut/coconutphp/blob/master/src/API.php#L35