Refactor store to return errors if problems and skip store.

Returns empty list if no problems.

Operations of data which validate or change records will return lists of
tuples
Tests for auto id stuff handling tuple response
Code and doctest fixes for returning tuples

Also slightly better error messages

Tests data set store tests should now pass.
Add mock handling store and returning errors to write integration tests

Should now be ready to refactor write to handle the new response from
store

This also expects incorrect headers to return an list of one errors.
This should help us handle things consistently. It begs the question
though. Should empty and create also return lists of errors? - will
entail more changes than just backdrop and pp-admin.
Refactor write to handle error list rather than validationerror

Still catch no data exception separately

Also returns error list for incorrect headers
pep 8 suggested fix

This reads slightly more clearly.