Provide a message to the end user when the authentication token fails

Ready for review

Tested and confirmed this does what it says on the tin.

The actual error I got was:

doTokenRequest() error: Client error: `POST https://login.eloqua.com/auth/oauth2/token` resulted in a `400 Bad Request` response: {"error":"invalid_grant","error_description":"The refresh token is incorrect, malformed, expired, or has been invalidate (truncated...)

But this is fine enough for me.

I'm calling this ready on the API side.

On the webform_eloqua handler we can add better messaging in #3440055 ✨ Show Eloqua Form Name in Webform Handler Summary Needs review

Merged into 2.0.x.

Automatically closed - issue fixed for 2 weeks with no activity.