[solution-import] add more descriptive error message when a solution environment variable is missing

Hi,

I had an issue where my unmanaged solution import failed with the very undescriptive message "Error: The given key was not present in the dictionary.".

After a lot of testing, I think I have isolated the error to be related to not having defined a deployment setting file, containing the required environment variable for my solution.

Here is my step-config; adding the last two lines resolved the error:

 - name: Import solution as unmanaged to build env
        uses: microsoft/powerplatform-actions/import-solution@v0
        with:
          environment-url: ${{inputs.BUILD_ENVIRONMENT_URL}}
          app-id: ${{inputs.CLIENT_ID}}
          client-secret: ${{ secrets.envSecret }}
          tenant-id: ${{inputs.TENANT_ID}}
          solution-file: ${{ inputs.solution_outbound_folder}}/${{ inputs.solution_name }}.zip
          force-overwrite: true
          publish-changes: true
          run-asynchronously: true
          use-deployment-settings-file: true
          deployment-settings-file: "env.json"

It took me some time to isolate the cause of this error, and I think it would be great to include a more descriptive error message.

The origin of the error message seems to be the target environment itself, so I guess an immediate fix would to be to add some client-side validations on the solution-import action, checking the contents environmentvariabledefinition.xml.

Start at the solution-import action and trace how deployment settings and environmentvariabledefinition.xml are handled during import. Confirm the missing-environment-variable failure path and define a descriptive client-side error; done means the action reports which required setting is missing instead of the dictionary error.