PXD-455 ⁃ isolate checking for validity of state/input

[rudyardrichter]

As is there is a lot of code interspersed throughout UploadTransaction and UploadEntity which checks to make sure that the input documents provided are correct and that the transaction is in a valid state (e.g. no nodes exist yet for entities to be created, or project id for nodes to be updated match the same project id for the transaction, etc.).

The code doing these checks should be separated from UploadTransaction and as self-contained as possible, to make more obvious/easier to find what is expected at any stage of the transaction.