exceptions in CAPTCHA generation in production

to investigate, possibly to pass down to @stephen once we have traces:

09:31:30 -ALERTOR1:#tor-alerts- DjangoExceptions[donate_neo/warning] alert is firing, 1 alerts on donate-01.torproject.org
09:36:30 -ALERTOR1:#tor-alerts- DjangoExceptions[donate_neo/warning] alert is resolved, 1 alerts on -donate-01.torproject.org
10:32:30 -ALERTOR1:#tor-alerts- DjangoExceptions[donate_neo/warning] alert is firing, 1 alerts on donate-01.torproject.org
10:37:30 -ALERTOR1:#tor-alerts- DjangoExceptions[donate_neo/warning] alert is resolved, 1 alerts on -donate-01.torproject.org