Add catalog item validation before attempting to install or uninstall #95
Description
Currently, if a catalog item does not have an Installer or Uninstaller defined, but is defined in a manifest, Gorilla will still attempt to download the installer, even though the URL is nil.
This results in confusing warnings that make it seem like Gorilla is attempting to download the entire repo and then fails to validate the hash of the entire cache directory.
process.Installs(), process.Uninstalls(), and process.Updates() should all verify that hash, location, and type are not nil.