Inform when the referenced configuration is invalid

[inqueue]

Add a more helpful message to indicate when the referenced configuration cannot be found.

❯ esrally list races --configuration=put-anything-here

    ____        ____
   / __ \____ _/ / /_  __
  / /_/ / __ `/ / / / / /
 / _, _/ /_/ / / / /_/ /
/_/ |_|\__,_/_/_/\__, /
                /____/


No recent races found.

In this example, ~/.rally/rally-put-anything-here.ini does not exist.

[siddhant1729]

Hi, I've been working on this issue and have a fix ready. Would love to submit a PR ..is this still open for contribution?