It could be really useful to let the user always see the error behind the invalid JSON reply, especially if related to the server configuration (see e.g. PHP file size limit) and not to a Moodle issue:

• commented out:

  moodle-master/lib/form/filemanager.js:

  ...

  } catch(e) {

  // TODO display error

  scope.print_msg(M.str.repository.invalidjson, 'error');

  //scope.display_error(M.str.repository.invalidjson+'<pre>'+stripHTML(o.responseText)+'</pre>', 'invalidjson')

  return;

  }

  ...

• managed:

  moodle-master/repository/filepicker.js:

  ...

  } catch(e) {

  scope.print_msg(M.str.repository.invalidjson, 'error');

  scope.display_error(M.str.repository.invalidjson+'<pre>'+stripHTML(o.responseText)+'</pre>', 'invalidjson')

  return;

  }

  ...