_project::_find_project_dir: fix error message

Now that we are also looking for WORKSPACE_PROJECT_FILE, we should add
it to the error message. While we're there, also mention the directory
we are looking in - this could help scripters be clear about what went
wrong.

The new error message looks like this:

    Error loading project: None of ['project.conf', '.bstproject.yaml']
    found in '/src/temp/blah' or any of its parent directories