If the tmp directory doesn't have permissions set correctly you give an error page. It starts with the message saying which user the web site is running as:

Piwik couldn't write to some directories (running as user 'apache').

But then appears to have a hardcoded www-data user and group:

Try to Execute the following commands on your server, to allow Write access on these directories:

chown -R www-data:www-data /var/www/html/piwik

It would be a lot better if you pulled the user and group from the system.