Widget API raises an error if query parameters are removed

[michaelwood]

Widget API raises an error if query parameters are removed on /search.widgets_api
'start'

If someone manipulates the URL for the widgets

KeyError: 'start'
  File "django/utils/datastructures.py", line 76, in __getitem__
    list_ = super().__getitem__(key)

MultiValueDictKeyError: 'start'
(2 additional frame(s) were not displayed)
...
  File "grantnav/frontend/views.py", line 602, in search_wrapper_xframe_exempt
    response = search(request, template_name)
  File "grantnav/frontend/views.py", line 732, in search
    return widget_search(request, json_query)
  File "grantnav/frontend/views.py", line 608, in widget_search
    start = int(request.GET['start'])