At present, we go right ahead and submit new records whose barcodes are not unique, and get back a 400 Bad Request with body "Barcodes must be unique, 12 is already assigned to another item" (which admittedly is nice and explicit.) But we shouldn't even submit such records.
This uses the okapi prop passed into the top-level components, rather than Okapi parameters pulled in from stripes-loader like the async "username" uniqueness validation in ui-users. We should backport this cleaner method to ui-users.
As we do for username uniqueness in ui-users
At present, we go right ahead and submit new records whose barcodes are not unique, and get back a 400 Bad Request with body "Barcodes must be unique, 12 is already assigned to another item" (which admittedly is nice and explicit.) But we shouldn't even submit such records.