Add event projection failure scenario when using "Update"

[Hawxy]

Unsure if this is related to community reported issues, but whilst building out a potential reproduction I ran into this. ops.Store works fine but ops.Update fails. The observations I've made is that the operations & batch execution appear in the correct order. The last operation in a batch ends up throwing a Marten.Exceptions.NonExistentDocumentException

[jeremydmiller]

@Hawxy I'd rather beat that with documentation that strongly suggests always using Store() inside of projection code.