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/format/IIIFInfo.js

@@ -89,10 +89,8 @@ function getComplianceLevelOfImageInfoForVersion(imageInfo, version) {
         && typeof imageInfo.profile[0] === 'string') {
         return imageInfo.profile[0];
       }
-      // TODO error: cannot get compliance level URL / string
-      break;
+      return;
     default:
-      // TODO error: invalid Image API version
   }
 }
 
@@ -111,11 +109,10 @@ function getVersionOfImageInfo(imageInfo) {
       if (getComplianceLevelOfImageInfoForVersion(imageInfo, Versions.VERSION1)) {
         return Versions.VERSION1;
       }
-      // TODO error: can't detect Image API version
       break;
     default:
-      // TODO error: can't detect Image API version
   }
+  assert(false, 61);
 }
 
 function getLevelProfileForImageInfo(imageInfo) {