Throw an err when we don't have hexo in package.json

[sunbohong]

As https://github.com/hexojs/hexo-cli/issues/249 mentioned, this commit hopes to add a prompt to facilitate the user to quickly identify the root cause of the problem

Before:

After:

[sunbohong]

Unit tests is expired.

https://github.com/hexojs/hexo-cli/pull/250/checks?check_run_id=1204740703 "The logs for this run have expired and are no longer available"