Our product is a web application. From time to time, we get failures, bugs, error etc and I’m looking to send out an email notification to inform our users what happened and how we dealt with it.
Should include things like:
Does anyone have a good template that they use for this?
I think you should treat this as part of incident management, and think about what you want this message to do for your users. From the question, you seem to be looking for boilerplate text, and I don't think that's the most appropriate - you may end up with some conventional meaningless wordage that just irriates the users more.
There's a conventional pattern to responses - expressing regret at the impact on the users, describing the response you're going to be making, possibly the reason for the incident.
In other words, a template for the sections it should contain is a good idea, a template of phrases to use, less so.
So I think you should
Express your regret for the incident occurence.
Describe the incident, whether it is still ongoing, and its cause, if known
Describe the impact on your users
Give the date/time when the incident started (or was detected)
Give the date/time on which it was resolved, if it has been
Indicate whether it is fully resolved, or worked around
Indicate what activities are stil underway, if any, and any expected timescales
Indicate when the message will next be updated (and update it then!)
Google for "incident management" "regret" "response" for some ideas.