Implement widget error handling

[mstijak]

It's highly probable that some fetch calls will fail. In that case, the widget should display an error message and offer a button to retry loading of the data. This should be implemented on the app level, instead of requiring that each widget implements it on its own.

[mstijak]

Unfortunately, global error handling is not very simple to implement.
Basic error handling is implemented in the btc-price-bchain-info widget and that can be used across widgets until a better way is found.