[cache] fix a cache issue with an entity created without relation cache prefill Prefilling the relation cache has its benefits sometimes, but for massive imports it may be a memory hog. The caches should be kept coherent even if there was no prefill. Closes #3828155.

[web/test] add a test for some http response codes Related to #3648809

[views] remove reference to the MAPPING_CHECKERS dict (closes #3810219) Was removed by by the changeset 65d93a4fd11c ([multi-sources-removal] Drop pyrorql and zmqrql sources)

[web/ajax] Always return a json dict with a 'reason' key in case of ajax errors Sending sometimes a bare unicode string and sometimes a json-encoded dict seems like a bad idea.