More informative errors when package not updgraded

Add information to user on how to set path when using upgradeRoxygen or when package not yet upgraded

e.g.

Error: This directory doesn't look like a package: Ensure upgradeRoxygen(path = 'folder/containing/your/DESCRIPTION'

Error: Looks like this is your first time using roxygen2 > 4.0.0.
Please run roxygen2::upgradeRoxygen('path/to/your/package/').