Webservice handling of errors

When there are errors in several tabs we should get all errors and not only the first tab. We should know which tab reported those errors. It would be usefull to know which field triggered the error (But this can be tricky: a same field can trigger several errors, and in the case of a composite, which field should be referenced?)

(from redmine: issue id 4805, created on 2016-05-25, closed on 2017-05-18)