Add dedicated error types for each of the error statuses

[chriso]

Dispatch does a good job of categorizing errors automatically. In some cases though it doesn't have enough information to be able to choose the correct function call status.

For example, in the GitHub example, the function has to manually check for a 403 response when a rate limit has been exceeded, and has no way of signalling to the Dispatch handler that it's a throttling error rather than a 403 Forbidden error.

This PR adds new error types that users can use in these cases.