Unclear fatal error when JSX not configured in rescript.json

[cknitt]

When one forgets to configure the JSX settings in rescript.json and tries to compile a ReScript source file containing JSX, one gets the following error:

Fatal error: exception Failure("Pexp_jsx_element is expected to be transformed at this point")

Instead, the user should get a clearer and more actionable error message, maybe something like

Cannot compile JSX expression because JSX support is not enabled. Add"jsx" settings to rescript.json to enable JSX support.

[cometkim]

Random idea: if ReScript provides a default jsx runtime, we could provide a default config for jsx.