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.

doc/errors/index.md

@@ -220,3 +220,11 @@ Duplicate item added to a unique collection.  For example, it may be that you tr
 ### 59
 
 Invalid command found in the PBF.  This indicates that the loaded vector tile may be corrupt.
+
+### 60
+
+Missing or invalid `size`.
+
+### 61
+
+Cannot determine IIIF Image API version from provided image information JSON.