Fix extract.uncompress crash with unknown file types

We should return with an error when the file type is unknown, as we are
not going to be able to process the unknown archive. Trying to do so
would cause a crash.

pkg/extract/extract.go

@@ -56,7 +56,7 @@ func uncompress(tarball, targetDir string, fileFilter func(string) bool, showPro
 	case strings.HasSuffix(tarball, ".tar"):
 		filereader = file
 	default:
-		logging.Warnf("Unknown file format when trying to uncompress %s", tarball)
+		return nil, fmt.Errorf("Unknown file format when trying to uncompress %s", tarball)
 	}
 
 	return untar(filereader, targetDir, fileFilter, showProgress)