Enable and save log file when DEBUG_PROXY is enabled

[massimomarazzi]

I some case error_log is disabled on server level, and If you enable DEBUG_PROXY no log message are write, appear only "there was an error loading matomo.js" message. Also, for example, when using WordPress and WP_DEBUG is enabled, on debug.log nothing entry are write, because script js call matomo.php directly, and WordPress don't run. I think this scenario are reproducible with other CMS.