add error boundaries

[tomwayson]

https://reactjs.org/docs/error-boundaries.html

Probably at the top level of each route, and maybe around the nav?

Would be ideal if we could get rid of the catch for fetching items:

create-arcgis-app/src/routes/Items.js

Lines 50 to 56 in 53e2c4a

	.catch(e => {
	this.setState({
	error: e.message || e,
	results: null,
	total: 0
	});
	});

I also wonder what happens if there are JSAPI errors w/in an error boundary. Should test that and maybe explicitly add one around the map component.