fix: devtools build-and-test script fixes

[geetexe]

Summary

1. Fixed a grammatical error: "Extensions have been build" -> "Extensions have been built"
2. Fixed an incorrect comment label: "# Firefox Chrome extension" -> "# Test Firefox extension"
3. Added an error handler to main() so that unhandled rejections exit with a clear error message instead of crashing silently

How did you test this change?

These are non-functional changes (typo/grammar fixes and error handling).
The first two have no runtime impact.
The third one adds a .catch() handler to the main() call, which is a standard Node.js practice for async entry points.