Think about consistent pattern for widget composition and failure management

[apirogov]

Currently widgets are mostly assumed to be instantitated for data nodes and be "simple".

Technically, they can also be instantiated for group nodes and used to build arbitrarily complex sub-dashboards based on multiple subnodes.

For the general cases, we probably should provide a generic meta-widget that can be used to configure a more complex view of non-interacting widgets (rows, columns, etc.)

For interacting fancy widgets, this necessarily will be a custom-build widget, because one cannot describe a fancy interaction in a simple schema and also not connect them technically.

In both cases, e.g. there should be some simplified way to check e.g. that referenced nodes and widgets exist, etc.

And we still need to catch exceptions and show some dummy widget with the error message.