[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.

[web/ajax] don't override any status code with 500 in ajax_error_handler If the request produced an error such as Unauthorized or NotFound, core_handle sets an appropriate status_out. Which is then overridden by the ajax error handler.