Handle when graphql handler not found in cell prerendering

[dac09]

Background

Cell prerendering about to be introduced in Redwood V3 requires the presence of the graphql handler at the default location under api/src/functions/graphql.{js,ts}

This is because the actual handler is used to make graphql calls internally when prerendering cells.

What's the issue?

It's possible (and plausible) that users move or rename the graphql handler in their project. In this situation we want to warn users that we couldn't find the graphql handler, and revert to the old behaviour of just rendering the Cell's loading state.

Other things to explore

• Can we allow user's to override the path that prerendering uses to load the graphql handler?