tart pull: retry if we get URLError

[edigaryev]

No one is going to throw Retryable anyway.

This makes the logic similar to the tart push side of things:

tart/Sources/tart/OCI/Layerizer/DiskV2.swift

Lines 37 to 42 in cd0f238

	if error is URLError {
	print("Error: \(error.localizedDescription)")
	print("Attempting to re-try...")
	return .retry
	}

Resolves #944.