Show some helpful error when --run-game cannot work

Instead of just try to run UI with invalid argument to fail,
it will just fail with better error messages.

Fixes #327

src/main.rs

@@ -241,6 +241,10 @@ fn main() -> anyhow::Result<()> {
                     return Ok(());
                 }
 
+                LauncherState::PredownloadAvailable { .. } if run_game => {
+                    return Err(anyhow::anyhow!("Predownload available. Use --just-run-game to run anyway, or open the UI to download it"));
+                }
+
                 _ => ()
             }
         }