Core: Throw an error for invalid story format

[yannbf]

Issue:
When mistakenly a user writes a story such as:

export const Basic = <div>Hello</div>

rather than:

export const Basic = () => <div>Hello</div>

Storybook fails with an error that is not very helpful:

What I did

This PR adds a check in the client api that will break Storybook if the story received is not a function telling exactly which story is broken. It's a similar mechanism than the one that checks if storyName is not a string.

Here's how it looks like:

I personally find it quite useful that Storybook breaks entirely because it tells the users that there is a critical error given that their stories are broken and it forces them to fix the problem right away.

How to test

1 - check the branch
2 - yarn build client-api
3 - add this to any of the example stories:

export const Basic = <div>Hello</div>

4 - run Storybook (e.g. yarn start)

• Is this testable with Jest or Chromatic screenshots? no
• Does this need a new example in the kitchen sink apps? no
• Does this need an update to the documentation? no

[gaetanmaisse]

Useful and clean 💪🏻 👏🏻

[shilman]

Great fix Yann 👍 👍 👍