Mosaico with WordPress Unexpected errors
This is a bug - and it looks like it has been reported upstream:

https://github.com/veda-consulting/uk.co.vedaconsulting.mosaico/issues/160

The temporary fix is to add an Alias to your Apache or Nginx configuration. I added the following for nginx:

location /templates/versafix-1/ {
  alias /srv/longshore/platforms/ourpowerbase-d7-c4.7.19/sites/all/extensions/uk.co.vedaconsulting.mosaico/packages/mosaico/templates/versafix-1/;
}