Fix search in Override operation screen

Steps to reproduce:

1. follow the link http://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20DM-7.10/listContributions
2. enter a keyword and click the Search button
3. an error page is displayed with this text

   {"entity-type":"exception","code":"javax.ws.rs.WebApplicationException","status":500,"message":"com.sun.jersey.api.MessageException: A message body writer for Java class org.nuxeo.ecm.webengine.WebException, and Java type class org.nuxeo.ecm.webengine.WebException, and MIME media type text/html was not found.\n"}