Notification when Home Assistant is unreachable

**Is your feature request related to a problem? Please describe.**
No.

**Describe the solution you'd like.**
If the app can't connect to the Home Assistant instance, send a notification. Smartthings did this (covered power and internet outages) and I just realized Home Assistant doesn't. It can be optional in the settings since some people wouldn't care, but I like to know if there's an issue at home and start a timer on my refrigerated items.

**Describe alternatives you've considered, if any**
None.

**Additional context**
None.