Allow hot-reloading on first compilation if .spv doesn't exist at load time

[ProfLander]

Currently, AssetServer gives up on loading an asset if it doesn't exist when load is called.

Thus, hot-reloading will fail for .spv files that don't exist at boot time. This is at odds with the rest of the hot-reload workflow, since you can go from fallback shader -> compiled shader if the .spv starts out empty, but will be stuck with the fallback shader until restart if the .spv starts out nonexistent.