Email Alert Callback status in Alert Callback History shows green check even if sending process times out

## Expected Behavior

When sending an email alert callback, if the firewall drops the connection or if there is any problem with the internet connection to the SMTP server, the Alert Callback History (aka Triggered alerts) should show a red cross near the "Email Alert Callback" entry when we select "Show callbacks".

## Current Behavior

Instead, a green check mark is displayed giving an illusion that the alert was indeed sent successfully. There is a system wide warning notification generated. But it is generated only for the 1st time the callback failed. There is no way to find out which alert was sent and which was not, how long the email alert outage issue existed.

## Steps to Reproduce (for bugs)

1. CentOS, iptables, graylog with SMTP configured and working.
2. Remove `iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT` from `/etc/sysconfig/iptables`
3. Reload iptables
4. Do something to trigger an email alert.
5. Check "Triggered alerts" section of the alert.

## Context

We had an incorrect iptables config which essentially removed the rule to allow ESTABLISHED and RELATED connections. We stopped receiving alerts in email and there was only one error notification in the "System / Overview" page with the time of the first failed email alert. We have had some 20 odd alerts after that but all show a green tick mark in the "Alert Callback History" giving a feeling that nothing was wrong with that instance of the callback.

## Environment

* Graylog Version: 2.1.1+01d50e5
* Operating System: CentOS 7
* Browser version: 54.0.2840.71 m (64-bit)