failure to copy SSL certificates should abort tasks

If aegir doesn't have permissions to read the SSL certificates, it just emits a warning and keeps on going. This affects at least migrate and verify.

In my opinion, this should provoke a full failure as it could disable a site during migration.

Extract from the verify task log:

Generated config encrypted virtual host configuration
SSL Certificate directory for example.com on ceres path /var/aegir/config/server_master/ssl.d/example.com exists.
SSL Certificate directory for example.com on ceres ownership of /var/aegir/config/server_master/ssl.d/example.com has been changed to aegir.
SSL Certificate directory for example.com on ceres permissions of /var/aegir/config/server_master/ssl.d/example.com have been changed to 700.
SSL Certificate directory for example.com on ceres path /var/aegir/config/server_master/ssl.d/example.com is writable.
WD php: copy(/var/aegir/config/ssl.d/example.com/openssl.key): failed to open stream: Permission denied in /srv/aegir/.drush/provision/provision.file.inc on line 38.
apache on ceres could not be restarted. Changes might not be available until this has been done. (error: Syntax error on line 18 of /var/aegir/config/server_master/apache/vhost.d/www.example.com: SSLCertificateKeyFile: file '/var/aegir/config/server_master/ssl.d/example.com/openssl.key' does not exist or is empty)
copy(/var/aegir/config/ssl.d/example.com/openssl.key): failed to open stream: Permission denied in /srv/aegir/.drush/provision/provision.file.inc on line 38.
Cleared all caches