Is it possible to display the admin bar while in the Theme Customizer?

Is this what you are looking for?

add_filter('show_admin_bar', '__return_true');

Codex

Btw following code in wp-config.php could help also:

define( 'WP_DEBUG', true ); // Or false
   if ( WP_DEBUG ) {
   define( 'WP_DEBUG_LOG', true ); // writes errors down in wp-content/debug.log
   define( 'WP_DEBUG_DISPLAY', true );  // shows errors on screen output, set false to on write into logfile
   @ini_set('display_errors',1); // set to zero to disable displaying it and only writing to logfile
   define('SAVEQUERIES', true); // will have a performance impact  so disable if not needed
   define('SCRIPT_DEBUG', true); // Use dev versions of core JS and CSS files (only needed if you are modifying these core files)
}