Warn on invalid config format

[m-allanson]

Description

Gatsby does not support using functions in its config, but it does not complain if you do. This can result in silent failures that are difficult to debug.

Possibly related PR: #16027

Steps to reproduce

cc @sidharthachatterjee do you have an example that can go here?

Expected result

Gatsby should complain that your config is invalid.

Actual result

Gatsby does not complain, instead unrelated errors appear later in the build process.

Environment

n/a

[github-actions]

Hiya!

This issue has gone quiet. Spooky quiet. 👻

We get a lot of issues, so we currently close issues after 30 days of inactivity. It’s been at least 20 days since the last update here.
If we missed this issue or if you want to keep it open, please reply here. You can also add the label "not stale" to keep this issue open!
As a friendly reminder: the best way to see this issue, or any other, fixed is to open a Pull Request. Check out gatsby.dev/contribute for more information about opening PRs, triaging issues, and contributing!

Thanks for being a part of the Gatsby community! 💪💜

[github-actions]

Hey again!

It’s been 30 days since anything happened on this issue, so our friendly neighborhood robot (that’s me!) is going to close it.
Please keep in mind that I’m only a robot, so if I’ve closed this issue in error, I’m HUMAN_EMOTION_SORRY. Please feel free to reopen this issue or create a new one if you need anything else.
As a friendly reminder: the best way to see this issue, or any other, fixed is to open a Pull Request. Check out gatsby.dev/contribute for more information about opening PRs, triaging issues, and contributing!

Thanks again for being part of the Gatsby community! 💪💜

[wardpeet]

We're launching a new api soon. It will give proper error messages based on Joi schema.