Display a notification if there is unsent message(s)

### This issue is: Ready to be Built

User may not be aware that a message they have sent is blocked on the device, because of an error, because of no network coverage, etc.

This is what WhatsApp is displaying in such circumstances:

## Solution
We show an Android Notification.

### Content
If cause is due to network connection:
> Some messages not sent
> Check your connection. Messages will send automatically when reconnected.

If the user needs to select Retry:
> Message not sent
> There was a problem sending your message. Please try again.

If cause unknown:
> You message was not sent
> Please try again.