Add prefix to the error message
You can override the output by implementing theme_status_messages in your theme.

You could work from something like this. See $myprefix that I've added.

function MYTHEME_status_messages($variables) {
  $display = $variables ['display'];
  $output = '';

  $status_heading = array(
    'status' =&gt; t('Status message'),
    'error' =&gt; t('Error message'),
    'warning' =&gt; t('Warning message'),
  );
  foreach (drupal_get_messages($display) as $type =&gt; $messages) {
    $myprefix = $type == 'error' ? 'ERROR: ' : '';
    $output .= "&lt;div class=\"messages $type\"&gt;\n";
    if (!empty($status_heading [$type])) {
      $output .= '&lt;h2 class="element-invisible"&gt;' . $status_heading [$type] . "&lt;/h2&gt;\n";
    }
    if (count($messages) &gt; 1) {
      $output .= " &lt;ul&gt;\n";
      foreach ($messages as $message) {
        $output .= '  &lt;li&gt;' . $myprefix . $message . "&lt;/li&gt;\n";
      }
      $output .= " &lt;/ul&gt;\n";
    }
    else {
      $output .= $myprefix . reset($messages);
    }
    $output .= "&lt;/div&gt;\n";
  }
  return $output;
}