Prevent "Saved data" view from raising an exception if a field is not found

[frapell]

fixes gh-381

[fredvd]

@frapell Looks good to me. But can we add a test for this? I think we can just duplicate the test linked below and add a "stalefield" to the test at saver.showFields = assignment. With your fix it should no longer crash.

collective.easyform/src/collective/easyform/tests/testSaver.py

Lines 137 to 162 in 5251149

	def testSaverDataFormShowFields(self):
	"""test saver data form show fields"""
	self.createSaver()
	self.assertTrue("saver" in get_actions(self.ff1))
	saver = get_actions(self.ff1)["saver"]
	self.assertEqual(saver.itemsSaved(), 0)
	request = FakeRequest(
	topic="test subject", replyto="test@test.org", comments="test comments"
	)
	saver.showFields = ("topic", "comments")
	saver.onSuccess(request.form, request)
	view = self.ff1.restrictedTraverse("@@actions")
	view = view.publishTraverse(view.request, "saver")
	view = view.publishTraverse(view.request, "data")
	view.update()
	form = view.form_instance
	message = form.description()
	self.assertEqual(message.mapping, {"items": 1})
	item = form.get_items()[0]
	self.assertEqual(item[1]["id"], item[0])
	self.assertEqual(item[1]["topic"], "test subject")
	self.assertEqual(item[1]["comments"], "test comments")
	self.assertTrue("replyto" not in item[1])