How can I turn off Extension Download Errors?
To do so, you need to add this following line on your civicrm.settings.php

global $civicrm_setting;
$civicrm_setting['Extension Preferences']['ext_repo_url'] = false;


https://wiki.civicrm.org/confluence/display/CRMDOC/Extensions#Extensions-Disableautomaticinstallationsofextension