wes4store model.on(datachanged): honor `return false` in event handlers

* fix `view` check for early stage where grid isn't binded yet

Signed-off-by: Oleg Verych <olecom@gmail.com>
App.backend.req: `xhr.responseText` in `json.err`

`xhr.responseText` has backend info which can be used to create
informative + l10n error messages

`xhr.statusText` is standard and don't suit for this

Signed-off-by: Oleg Verych <olecom@gmail.com>