Allow disabling email alerts
When a polling error happens, the Kresus server will send an email on every polling attempt, which is a bit annoying. Alternatively, it could just send an email the first time, and let the user know when the error has been fixed. Since some users did want to get the error email every time it'd happen, we could consider allowing repeating the error email alert (it'd be disabled by default, to avoid spamming).