added error-response when running request with XHR

[oleg-andreyev]

Subject

Adding error-response for XHR requests

Current behaviour: when submitting form with AJAX (XHR) and if error occurs HTML will be returned instead of JSON.

I am targeting this branch, because features (BC?)

Changelog

### Changed
Added JSON response from edit/create action when form is invalid.

To do

• Update the tests

[greg0ire]

Adding error-response for XHR requests

Please describe the current behavior. Do we currently return an empty response?

[oleg-andreyev]

Adding error-response for XHR requests

Please describe the current behavior. Do we currently return an empty response?

Updated description

[greg0ire]

if error occurs HTML will be returned instead of JSON

Do you think people might currently be relying on that kind of HTML?

[oleg-andreyev]

@greg0ire I think it’s possible but only as part of workaround for this inconsistent behavior.

[greg0ire]

Maybe we should apply your fix only when the client sends an Accept header that requests application/json, and trigger a deprecation otherwise? The deprecation would say that next major will reject text/html with a 406

[oleg-andreyev]

@greg0ire @core23 @OskarStark I've done what @greg0ire suggested above, I've done same for success result.

[core23]

Thansk @oleg-andreyev

[fancyweb]

This PR causes an issue with SonataPageBundle since this bundle uses the CRUDController and does all edit with ajax requests. Everytime validation fail on a block edit, an error flash message is added (contrary to before). Then all those messages are displayed at once on the next page the user visits. What can we do about this?

[OskarStark]

Can you share a screenshot?