[views] implement a custom json error view If the client asked for ``(e)jsonexport`` view, answer him with a json response rather than an error formatted in a big HTML blob. add a custom _requested_vid predicate. In cw 3.21, it will be deprecated in favor of ``match_form_params(vid=('jsonexport', 'ejsonexport'))``

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile