add backup targets table

src/backupformat.js

@@ -4,9 +4,13 @@ exports = module.exports = {
     api
 };
 
+const BoxError = require('./boxerror.js');
+
 function api(format) {
     switch (format) {
     case 'tgz': return require('./backupformat/tgz.js');
     case 'rsync': return require('./backupformat/rsync.js');
     }
+
+    throw new BoxError(BoxError.INTERNAL_ERROR, `Undefined format ${format}`);
 }