Warn the user that the dev_tools installation directory is not owned by the current user

I installed dev_tools to /opt/dev_tools, and found that it wasn't working. I later discovered that it uses Git to update dev_tools, and thus required write access from the current user. To fix this, I changed the ownership of /opt/dev_tools to be for my user, but I thought cr should warn the user if the directory is not owned in the first place. It may not be perfect, and there is probably a better solution, but this is a good option for now in my opinion :)