***cachedResponseDictionaryForRequest:key:handler failed:
Error Domain=NSCocoaErrorDomain Code=4099
"The connection to service named com.apple.nsurlstorage-cache was invalidated."
https://developer.apple.com /library/prerelease/content/documentation/Xcode/
Conceptual/swift_playgrounds_doc_format/index.html
Playground Book Package Format