[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'))``

if repo.system_source.dbdriver == 'postgres':
    sql('ALTER TABLE appears ADD COLUMN weight float')
    sql('UPDATE appears SET weight=1.0 ')