WebSearch: fix for double display of the footer
WebSearch: fix for double display of the footer

* Raises an exception in the case there are no results
  to display in order to avoid displaying twice the footer
  and the box proposing links to other useful search engines.

* Removes calls to page_end() and
  perform_external_collection_search_with_em() when there are no
  results to display since these functions will be executed
  in the top level.

* Handles the exception to postpone the execution of those
  functions.

Co-authored-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio