chore: (minor) Add better error messaging for CLI and improve quickstart docs

[aditya-67]

This PR introduces better error messaging for CLI commands and improves the Quickstart docs

• Check for environment variables before executing runs

After

• Check for output.json before running the ui command. UI opens with a loading screen and terminal throws a file not found error

After

• Quickstart docs - Add a line to create a .env with OPENAI_API_KEY to make sure the first run is always successful than showing an error message AI202: process.env.OPENAI_API_KEY is not set

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 672267674a36a21455ede41fe9b7bd65ff0af721

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------------- | ----- | | @empiricalrun/cli | Patch |

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[saikatmitra91]

Amazing @aditya-67! Thanks for submitting PR ❤️
Will take a look in a while.