Support rendering a fallback if a child fails to render

Problem/Motivation

When rendering a render array, we recursively call doRender on the renderer.
However there is only a single try/catch at the root render call (in ::render) so a child render element failure can cause the whole render array to output nothing.

Steps to reproduce

Proposed resolution

Add support for #fallback or similar to render arrays
Wrap the recursive ::doRender calls in the renderer in try catch and render the fallback in case of a failure.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet