Add data-testid to test relevant elements (fixes #244)

Change-Id: Id908e38f01e426543ed98975544204a9a632294d
diff --git a/templates/exception.html.ep b/templates/exception.html.ep
index f85edc6..abff9aa 100644
--- a/templates/exception.html.ep
+++ b/templates/exception.html.ep
@@ -1,6 +1,6 @@
 % my $msg = stash('msg') // stash('exception')->message // '500: Internal Server Error';
 % layout 'main', title => 'KorAP: ' . $msg;
 
-<p class="no-results"><%= $msg  %></p>
+<p class="no-results" data-testid="error-message"><%= $msg  %></p>
 
 % notify('error' => $msg);