By default the stack trace is pretty unreadable, so you might want to wrap it in <pre> tags. Here I also wrap it in a <div> and set the class 'alert alert-danger' which are CSS classes in the Bootstrap CSS framework to style them red.
<?php
function exception_handler($exception) {
echo '<div class="alert alert-danger">';
echo '<b>Fatal error</b>: Uncaught exception \'' . get_class($exception) . '\' with message ';
echo $exception->getMessage() . '<br>';
echo 'Stack trace:<pre>' . $exception->getTraceAsString() . '</pre>';
echo 'thrown in <b>' . $exception->getFile() . '</b> on line <b>' . $exception->getLine() . '</b><br>';
echo '</div>';
}
set_exception_handler('exception_handler');
