Improve "no valid rules" error message to include file type

[OliverJAsh]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

When adding tslint to a project I was confused by the error message "No valid rules have been specified", even though I did specify valid rules, and those rules were working.

It turns out this was because I had not defined any jsRules and I was linting JS files.

This PR improves the error messaging to make that clearer.

Is there anything you'd like reviewers to focus on?

CHANGELOG.md entry:

[palantirtech]

Thanks for your interest in palantir/tslint, @OliverJAsh! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[OliverJAsh]

Any updates on this?

Build is failing with:

/bin/bash: yarn: command not found
Exited with code 127

[suchanlee]

@OliverJAsh can you try merging master into your branch and see if we can get the build to run?

[OliverJAsh]

Hi @suchanlee, done!