Provide filled instance to host application for submission

[lognaturel]

Depends on #82

User Story

As an application developer
I want to get access to filled forms from web forms
So that I can submit them to a server or otherwise process them

As a form end user
If my submission fails, I want to know what happened (e.g. I'm offline, the server isn't responding, etc)
So that I can take appropriate action

[lognaturel]

I think we'll need to have two-way communication, here. That is, if the host application gets back some kind of error when it tries to do whatever it wants with the submission, it feels like that should be passed back to web forms (if web forms owns the "container", at least). I tried to capture that need in an additional user story.