GdsApiProxy for "tell don't ask" no raise APIs

Proxy object for wrapping GdsApiAdpaters.
Presents a promise-like interface with success and error callbacks.
Whilst provision is made to allow on_error callbacks to be provided to the GdsApiProxy, we don't actually *use* them for anything at the moment.

Previously, when the GdsApi was timing out/erroring, we at least got error notifications and extra information in the logs. Now, it doesn't appear that we do anything :(

Shouldn't we be at least logging the error and/or manually raising to errbit?