add 'webp' to list of available formats in svgToImg error msg

Author: etpinard

src/snapshot/svgtoimg.js

@@ -89,11 +89,12 @@ function svgToImg(opts) {
                     imgData = url;
                     break;
                 default:
-                    reject(new Error('Image format is not jpeg, png or svg'));
+                    var errorMsg = 'Image format is not jpeg, png, svg or webp.';
+                    reject(new Error(errorMsg));
                     // eventually remove the ev
                     //  in favor of promises
                     if(!opts.promise) {
-                        return ev.emit('error', 'Image format is not jpeg, png or svg');
+                        return ev.emit('error', errorMsg);
                     }
             }
             resolve(imgData);