Display an error if propType value is a reference to a variable when building backend components wuth dash-generate-components

[HammadTheOne]

As pointed out in plotly/dash-bio#564, if we reference a variable within the react.js source code for a component propType, the components are generated for Python and R, but missing the values themselves when mapping from the PropTypes js type object to the Python/R type. For Julia, the components fail to generate with a type-error.

As @alexcjohnson suggested, it makes more sense to have the component generator throw an error when this happens so that component authors can fix this kind of problem up front rather than expecting them to notice a problem later and fix it.

[gvwilson]

@HammadTheOne is this still a concern? thanks - @gvwilson

[HammadTheOne]

No, we can close it for now.