Corretly report 409 conflict when collection creation is cancelled on error but other resources started to point to it in the meantime.

[zozlak]

It is possible that during the collection creation which will fail at the end (especially on a handle as this happens very late) another resource is created which points to it. In such a case the call to RC::$transaction->deleteResource($this->id); in the Resource::postCollectionMetadata() throws a database foreign key check violation error which leads to a cryptic HTTP 500 while it should be reported as an original error which lead to the resource creation cancellation.

[zozlak]

Actually it's a very good question what should be reported in such a case...

[zozlak]

Resolved by the 590959c