Update error message on failing to find hostpolicy for self-contained

[elinor-fung]

• Specify if the config wasn't found versus didn't have a framework
• Show message for apphost (not just dotnet)

No runtimeconfig:

Failed to run as a self-contained app.
  - The application was run as a self-contained app because '<...>' was not found.
  - If this should be a framework-dependent app, add the '<...>' file and specify the appropriate framework.

No framework specified:

Failed to run as a self-contained app.
  - The application was run as a self-contained app because '<...>' did not specify a framework.
  - If this should be a framework-dependent app, specify the appropriate framework in '<...>'.