Issue #2805281 by Wim Leers, damiankloip: ?_format=hal_json error responses are application/json, yet should be application/hal+json

Issue #2739617 by dawehner, Wim Leers, neclimdul, chx: Make it easier to write on4xx() exception subscribers

Issue #2737719 by Wim Leers, dawehner: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method

Revert "Issue #2737719 by Wim Leers, dawehner: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method"

This reverts commit 59781ffc.

Issue #2737719 by Wim Leers, dawehner: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method