Improve error message while importing subproject

If you import a wrapped project with errors in its build definition will produce an error: Also couldn't find a fallback subproject in subprojects/<project_name> for the dependency <dep_name>

While the error could just be a wrong spelled source file in subprojects/<project_name>/*/meson.build. We expect it to say that the project has errors not that the project doesn't exist.

As sample: This project imports the subproject without error except that it doesn't build(not completed yet..). This project shows the error while I just added a wrong source file in the subproject build definition.