don't die if classname to filename mapping fails

If a command was defined in a file that does not map to its package name (for instance a file that defines multiple command packages), the method died. With this fix, the abstract is just "(unknown)".
add test for args processing in one-command app
changelog for abstract fix
v0.323

          fall back to "(unknown)" for command abstract in more cases (thanks,
          Jakob Voss!)