Fix Project Wiki update

It's showing Wiki was successfully updated. even with invalid params.

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/projects/wikis_controller.rb#L51 doesn't seem correct since it's checking upon a WikiPage object that's always being returned on WikiPages::UpdateService#execute. In another words, else will never be reached.

Steps to reproduce

1. Create an project wiki page
2. Update it with invalid params (no content)
3. Wiki page isn't updated but Wiki was successfully updated. is presented

Correct behavior

1. Create an project wiki page
2. Update it with invalid params
3. Invalid params error should appear

Thanks @dhu for bringing this up.