legacy: fix websearch unformatted vars stacktrace
legacy: fix websearch unformatted vars stacktrace

* Changes output generation of `perform_index` to use `.format()`.
  Requesting the admin interface page for WebSearch previously always
  resulted in a stacktrace due to unformatted strings.

Signed-off-by: Dimitrios Semitsoglou-Tsiapos <6a6862363aaa6e93bbf0fa721048d2ca97773c19@cern.ch>
Reviewed-by: Jan Aage Lavik <8d55e006a1b29770f275a5c59a7341f17f8e69d5@cern.ch>

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