problem with jsonification of dexterity validation errors

[tdesvenain]

in plone.rest we serialize exceptions with a string cast https://github.com/plone/plone.rest/blob/master/src/plone/rest/errors.py#L37

in plone.restapi, we raise BadRequest with a list of dictionaries as a parameter when we have dexterity fields validation error. https://github.com/plone/plone.restapi/blob/master/src/plone/restapi/deserializer/dxcontent.py#L114

this results in wrong json (with single quotes, unicode prefixes (u''), classes...)

i'm not sure how to fix this

• we could improve exception message rendering in plone.rest
• we could json.dump the dictionary list in dxcontent code. this is not very clean (two level of jsonification) but would not result in any breaking change

[tdesvenain]

cc @Gagaro

[tisto]

@tdesvenain I think we should try to improve the exception message in plone.rest.