Add i18n for modal network error dialog

[cbeer]

This error handling code is not localized:

https://github.com/projectblacklight/blacklight/blob/master/app/assets/javascripts/blacklight/modal.js#L108-L112

  var contents =  "<div class='modal-header'>" +
            "<div class='modal-title'>Network Error</div>" +
            '<button type="button" class="blacklight-modal-close close" data-dismiss="modal" aria-label="Close">' +
            '  <span aria-hidden="true">&times;</span>' +
            '</button>';

We should extract the modal title and close button to a localizable form (also, what is the happy path for localized javascript?)

[barmintor]

Please correct me if I misunderstand, but marking as 7.x since it could be handled in a feature release.