Added _create_save() method.

This passes along the form for saving, which can be customized by a
subclass.
Updated render functions to take status parameter.

The render functions can set the status for the requests.
Added errors to context.

This is a list of messages that are not HTML-formatted.
Updated _create_success() to handle json.

When the request format is JSON, return a structure with a success
message and the JSON-encoded item created.
Added status 400 to create error responses.
Removed form from ctx for rendering json error response.