Fix #925

If an object being created/updated contains a foreign key URI that is malformed, it is more correct to return 400 (bad request) than 404 (not found), as the latter incorrectly suggests that the target URI of the request was not found. This PR also contains two other related changes: one for when a foreign key URI matches multiple resources (such a request now returns 400 rather than 500), and one to fix a minor error in the multiple resource error message.