Remove content api from manual publishing pipeline

Manuals are no longer published to content api but instead published to content store via the publishing api. This is a first step towards removing content api.

with @tommyp
Export to publishing api before rummager

We should publish content to content store before publishing to search. This is so that when something arrives in search there is no way a user can reach the page before the content is available in the content store
Merge pull request #254 from alphagov/remove-content-api

Remove content api from manual publishing
Handle GdsApi HTTP error types

Instead of rescueing from specific HTTP errors raised by the panopticon registerer. Instead rescue from GdsApi::HTTPErrorResponse types
Reraise panopticon publication errors for worker

Re raise any errors created by the PanopticonRegisterer in order to catch them in the worker. This is something we would like to change. wrapper classes around the gds-api-adapters should handles errors and error logging themselves in the future. There is thought towards allowing the API wrappers to an object which stores the state of the publishing (PublishingTask maybe?) which could be handed to the worker in order to query the state of a particular publishing job. This would allow us to correctly raise an error in the worker without frivolously catching GdsApi exceptions and making the worker unaware off GdsApi.