Ensure action fails when used so users are pointed to updated docs

[AndrewKahr]

Changes

• Ensure action prints an error and informs the end user about updated activation instructions

Checklist

• Read the contribution guide and accept the code of conduct
• Readme (updated or not needed)
• Tests (added, updated or not needed)

[GabLeRoux]

I tested this and it worked as expected 👍

Documentation at v3 suggested a workflow like this:

name: Acquire activation file
on:
  workflow_dispatch: {}
jobs:
  activation:
    name: Request manual activation file 🔑
    runs-on: ubuntu-latest
    steps:
      # Request manual activation file
      - name: Request manual activation file
        id: getManualLicenseFile
        uses: game-ci/unity-request-activation-file@v2
      # Upload artifact (Unity_v20XX.X.XXXX.alf)
      - name: Expose as artifact
        uses: actions/upload-artifact@v2
        with:
          name: ${{ steps.getManualLicenseFile.outputs.filePath }}
          path: ${{ steps.getManualLicenseFile.outputs.filePath }}

Once merged, using the above and executing the manual action will produce output like in above screenshot.