Check for custom content presence

[DCxDemo]

Since the patch allows to run without content, gotta make sure it doesn't try to load anything missing. Basically gotta add a safeguard somewhere that will prevent content patching functions from running in case the content is missing.

Maybe also pack everything in an additional PKR file somehow, so can only distribute one file instead of a zip. wont require user to unpack it as well.

Alternatively, or until implemented, can let launcher validate it, and also let it unpack the zip, so user can just drop it into the root folder.