Improve the error message

[oravecz]

Developer experience request

Currently, if dataLoader.getItem() returns a null or undefined value, the message to the user is:

errors.js:2 Uncaught Error: Headless Tree: sync dataLoader returned promise
    at throwError (errors.js:2:40)
    at unpromise (feature.js:15:15)
    at retrieveItemData (feature.js:31:20)

Code for unpromise is located here.

Perhaps that can be changed to...

const undefErrorMessage = "sync dataLoader failed to return data value";
const promiseErrorMessage = "sync dataLoader returned promise";
const unpromise = <T>(data: T | Promise<T>): T => {
  if (!data) {
    throw throwError(undefErrorMessage);
  }
  if (typeof data === "object" && "then" in data) {
    throw throwError(promiseErrorMessage);
  }
  return data;
};

[lukasbach]

Updated, thanks for the suggestion!