Better structure

Cleaner and easier to maintain.
This was a string because I didn't want to throw this error as it's an expected error. See the CLI check: https://github.com/sindresorhus/pageres/blob/dd839b6ffd612e45d9a6c76febbf4443c98e5490/cli.js#L88-L92

Throwing the error prints the full stack, but I only wanted to show the message. Any better way to show only the string in the CLI for this, but throw on actual errors?