Export `WaspHttpError` from `wasp/client/api`

[infomiho]

The signup and login functions that are available to the user can throw the WaspHttpError, but users can't import this class. It's useful for users to import his class to be able check if a thrown error is an instance of WaspHttpError.

The error is thrown from the handleApiError fn: https://github.com/wasp-lang/wasp/blob/main/waspc/data/Generator/templates/sdk/wasp/api/index.ts#L110