* Allow more complex errors from plugins
This enables more complex types to be registered and returned from plugins.
* Register common error types
This is a slightly less drastic change, which keeps the HTTPCodedError
as an interface.
* Remove replication error from list
specify more concrete error cases to make their way back up the stack.
Over time there is probably a cleaner way of doing this, but that's
looking like a more massive rewrite and this solves some issues in
the meantime.
Use a CodedError to return a more concrete HTTP return code for
operations you want to do so. Returning a regular error leaves
the existing behavior in place.
