Integrate view-map into ordoor

Now we can view scenario maps from the main game interface. We can't
cancel out of a scenario yet, though.

internal/assetstore/assetstore.go

@@ -120,7 +120,7 @@ func (a *AssetStore) lookup(name, ext string, dirs ...string) (string, error) {
 		}
 	}
 
-	return "", os.ErrNotExist
+	return "", fmt.Errorf("file %q does not exist", filename)
 }
 
 func canonical(s string) string {