Fixes #80 by capturing the exception if the server is unreachable

This commit fixes #80, the problem by verifying if the search throws an
exception, properly handling it to return an empty list instead.
In addition, the rendering page will display the information about
which Registry URL is unreachable, so that helps admins to properly
handle the problem.

*	modified:   grails-app/controllers/docker/registry/web/RepositoryController.groovy
- Handling the case when the repositoryService.index fails because
  the server is unreachable. If that's the case, just return an
  empty list and print an error message for further debugging/monitoring.

*	modified:   grails-app/domain/docker/registry/web/Registry.groovy
- Adding a transient property "unreachable" for rendering purpose.
- Adding the transient method "setAsUnreachable" that's set in the controller.
- Adding the method implementation to just set it as unreachable.

*	modified:   grails-app/views/repository/index.gsp
- Printing the warning message "(Unreachable) to the header of the repository
  in case the server can't be reached.
@marcellodesales - Instead of an additional Map, can we add a decorator class that'll wrap the Registry instance that will hold if it's up/down? Slightly tidier I think.