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.
@atc- I just submitted a View class that holds both the Registry and the Repositories, along with the repository reach status at pull request #89. In addition, I have added the implementation details to the Service, so that it can be reusable by other controllers/service as you had pointed out before!

I will close this pull request in favor of pull reques #89.