Angular fetch updates needed

[tjvantoll]

See https://github.com/NativeScript/docs/issues/1274, but essentially we need to update our tutorial content to account for Angular API updates.

[sebawita]

refactored project:
https://play.nativescript.org/?id=gacaAW&template=groceries-ng&v=4

Updated Http => HttpClient
Added Error handling for when username and password are empty.
Changed services to be provided in root by default, so that there is no need to explicitly add them to their component providers.

[sebawita]

and now with an even better error messaging intercepted from Kinvey:
https://play.nativescript.org/?id=gacaAW&template=groceries-ng&v=5

see handleErrors() in both services