Error summary: ability to place on a nested form

[dlockhart]

In a complex application where form elements are spread out across multiple shadow boundaries, you still want to validate and show the error summary for the entire "page". This is possible today using nested <d2l-form>s, which is awesome. However, the error summary needs to be displayed with a form that wraps the entire page.

Ideally, we could choose which d2l-form hosts the error summary. That would allow us to handle cases like with the primary/secondary template where you have form elements in both panels but wish to display the summary in the primary panel.

In the example above, it would be good to be able to show the errors from both panels, but currently only the primary panel errors can be shown.