Simplify error handling in build-dashboard.js

[coderabbitai]

As discussed in PR #3304 (comment), @akshatnema agreed to improve the error handling by throwing errors directly instead of using Promise.reject(e); in the build-dashboard.js script.

We should update the error handling code to throw errors directly, making the asynchronous functions more idiomatic and easier to read.

[github-actions]

Welcome to AsyncAPI. Thanks a lot for reporting your first issue. Please check out our contributors guide and the instructions about a basic recommended setup useful for opening a pull request.
Keep in mind there are also other channels you can use to interact with AsyncAPI community. For more details check out this issue.

[lakshaydewan]

Working on same will be raising a PR soon..

[lakshaydewan]

PR #3310 fixes the issue needs a review though..

[yuvrajkarna2717]

Hello, I noticed this issue and would like to contribute. Is it still open for work?