fix: synchronous errors from addPhoto

[gmaclennan]

Other draft observation store actions throw an error synchronously if there is no current draft observation, however the addPhoto action was defined as an asynchronous function, so the error from asserting a draft exists first was thrown asynchronously.

This PR changes the implementation of addPhoto so that the initial addition of an empty attachment (before photo processing) is done synchronously, so that the error from a draft not existing is thrown synchronously. This matches the behaviour of the other actions.

It's not necessary to return a promise from addPhoto, since we don't have any code that uses this promise, but it could be useful in the future (the promise resolves when all the photo versions are created). It is important to catch any errors from photo processing, to avoid the risk of an uncaught error.

I'm open to a preference to just return void from addPhoto rather than Promise<void>.