More helpful error messages when maintainers haven't set their projects' types properly.

This came up in #1938200-5: Sandbox fails to launch with "The selected project shortname xxxxxxx could not be found."...

If a project type (mainly occurs with sandbox projects) is not set to either one of "Modules", "Themes", "Distributions", "Drupal core" and is instead specified as a "Drupal.org projects" type, then users trying to launch a simplytest.me sandbox of such projects get this error:

The selected project shortname xxxxxxx could not be found.

That does not help them troubleshoot the issue and the first thing to do is head here to report such similar issues. How about instead we also throw some tip about it like so:

The selected project shortname xxxxxxx could not be found.

A common reason for this is because sometimes maintainers happen to not properly set their project's type. If the project you are trying to test is a sandbox, then please report the issue to the maintainer and ask them to make sure that they have not set their project to be of type "Drupal.org projects".