SWR should throw an error when suspense is enabled but key is not ready

[shuding]

Related to #339. When the suspense option is enabled, SWR can not return any value to continue rendering, neither throwing any promise if it can't fetch. This includes 2 situations:

1. suspense: true and key is falsy.
2. suspense: true and fetcher is null.

In both cases SWR should throw an error.

[koba04]

@shuding
How are you going to add the error?
The easiest way to add this is like the following.

if (suspense && (!key || fetcher === null)) { 
  throw new Error("Either key or fetcher must be specified with suspense mode");
}

But I think SWR hasn't had any errors like this yet and its bundle size is important. Those error messages cannot be minified and would increase the bundle size. Is it acceptable for SWR?
Or are you going to adopt a way to avoid increasing its bundle size like what React and Redux do?

• Port error message extraction setup and values from master reduxjs/redux#4057
• https://github.com/facebook/react/tree/main/scripts/error-codes

I remember that there was a similar discussion before.
#898

[shuding]

I think maintaining a list of error codes like React is a good idea. 👍

And in longer term we need to create 2 builds, one for development (warnings, errors, useDebugValue) and one for production use (minimal bundle with only error codes).

[shuding]

That said I'm good with fixing this with the proper error message for now and address error codes later, correctness is more important than size. 👍