`/leap/train-model`: Show an error message for free LeapAI API users

[chinmaykunkikar]

Models cannot be trained if the user is using a non-subscribed version of LeapAI API. See: https://tryleap.ai/pricing

This is also referenced in a couple of issues:

• by @lok0919 in https://github.com/leap-ai/headshots-starter/issues/51#issuecomment-1752784754
• by @chinmaykunkikar in https://github.com/leap-ai/headshots-starter/issues/44#issuecomment-1756057983

Change details:

• Instead of users finding out about it at the last stage of local setup, warn them earlier before the Local Setup section of the README.
• Show error messages on the toast and on the console for better UX and DX.
  • Status 402 is about not having a premium API
  • Status 400 is about not having a proper webhook url
• Fixes a TypeError (see https://github.com/leap-ai/headshots-starter/issues/44#issue-1932049482)

Try it on LeapAI's official playground -- https://reference.tryleap.ai/reference/train

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