Fixes: #17498 - Avoid error in bulk import when non-unique identifying values are provided

[bctiemann]

Fixes: #17498

Extends _clean_fields to catch MultipleObjectsReturned exceptions on any field where an ad-hoc accessor is used (for example manufacturer.description in DeviceType) and multiple matching records are found.

With this change, a toast is displayed which surfaces the model that caused the exception. Because this is a non-field clean method, the field's specific metadata is not available, but the general message from the exception (which contains the model name) is reflected in the toast which should lead the user to a solution.

Note that other extraneous toasts appear as well as a side effect; this is not ideal, but this solution avoids the much greater issue of an opaque 500 error.

[jeremystretch]

I think the root issue here is that the manufacturer field on DeviceTypeImportForm should be a CSVModelChoiceField, which properly handles the MultipleObjectsReturned exception. (The form's default_platform field should also be corrected.)

That said, I like your approach overriding the _clean_fields() method. We should explore whether this would obviate the need for our custom CSVModelChoiceField, but that should be handled as a separate housekeeping issue to ensure consistent implementation.

[bctiemann]

Closing in favor of #17593