chore: better prettier error message

Description

This MR alters the npm run lint script slightly to explain what went wrong.

Inspired by !1493 (comment 1831055842)

Before

Checking formatting...
[warn] README.md
[warn] Code style issues found in the above file. Run Prettier to fix.
Cleaning up project directory and file based variables 00:00
ERROR: Job failed: exit code 1

After

job log: https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/jobs/6477253483

> prettier --check '**/*.{js,ts,mjs,vue,json,md}' || echo 'Prettier failed. Use "npm run autofix" and push the fix.' && exit 1
Checking formatting...
[warn] README.md
[warn] Code style issues found in the above file. Run Prettier to fix.
Prettier failed. Use "npm run autofix" and push the fix.
Cleaning up project directory and file based variables 00:01
ERROR: Job failed: exit code 1