ProductProvider needs to warn about missing variants

[frehner]

What is the location of your example repository?

No response

Which package or tool is having this issue?

hydrogen-react

What version of that package or tool are you using?

2023.1.7

What version of Remix are you using?

n/a

Steps to Reproduce

We need to have a better error/warning for this situation:

1. Use ProductProvider, passing a data prop that doesn't contain variants
2. Get a warning from flattenConnection that it couldn't flatten the connection

hydrogen/packages/hydrogen-react/src/ProductProvider.tsx

Lines 55 to 58 in 1440953

	const variants = useMemo(
	() => flattenConnection(product.variants ?? {}),
	[product.variants],
	);

Expected Behavior

ProductProvider should have a warning with details of what's missing on the product prop.

Actual Behavior

flattenConnection(): The connection did not contain either "nodes" or "edges.node". Returning an empty array.

[frehner]

Also need to do this for sellingPlanGroups and sellingPlanAllocations

In other words, any place that flattenConnection is used there.