only return an error when trying to load a different version of the JSAPI

Currently, we throw return an error when the user calls bootstrap more than once, regardless of:

• whether or not the first script tag is still loading
• the first script tag is for the same version of the JSAPI as subsequent script tags

Currently this is causing problems in ember-esri-loader tests (https://github.com/Esri/ember-esri-loader/pull/27). I can work around those problems in that library, but I am wondering if it actually makes sense to return an error in all the above conditions.

I’m pretty sure we want to return an error whenever the user is trying to load a different version of the JSAPI.

If it’s the same version of the JSAPI, and if the script has already loaded we should just call the callback w/ window.require as we do here:

https://github.com/Esri/esri-loader/blob/ff9989bdc42cfce8a3f30a2a977f45fa818317f1/src/esri-loader.ts#L47-L56

If the script hasn’t already loaded, I think we want to piggy back on the original onload call.

[tomwayson]

Also, whenever we return an error b/c a script is already loading/ed, we should also pass that script as the second argument to the callback as a convenience to the caller (I know that would be helpful to me right now as I’m working through how to handle the above errors in ember-esri-loader).

[tomwayson]

Also, I’m fairly certain that this line:

https://github.com/Esri/esri-loader/blob/ff9989bdc42cfce8a3f30a2a977f45fa818317f1/src/esri-loader.ts#L34

Needs to be guarded with an if (callback) check like we do here:

https://github.com/Esri/esri-loader/blob/ff9989bdc42cfce8a3f30a2a977f45fa818317f1/src/esri-loader.ts#L52-L57