Add error codes for IIIF errors

60: "Missing or invalid `size`."
Without at least a size, no IIIF can be displayed.

61: "Cannot determine IIIF Image API version from provided image
information JSON."
Without finding out the version information, one could only guess how to
use the image info JSON.

src/ol/source/IIIF.js

@@ -54,9 +54,8 @@ class IIIF extends TileImage {
     const version = options.version || Versions.VERSION2;
     const sizes = options.sizes || [];
     const size = options.size;
-    // TODO Appropriate error code
     assert(size != undefined && Array.isArray(size) && size.length == 2 &&
-      !isNaN(size[0]) && size[0] > 0 && !isNaN(size[1] && size[1] > 0), 999);
+      !isNaN(size[0]) && size[0] > 0 && !isNaN(size[1] && size[1] > 0), 60);
     const width = size[0];
     const height = size[1];
     const tileSize = options.tileSize;