show notification if error occurs

This is a proposal to show an error notification if the VPN stopped due to an error. Sometimes it's hard to notice if there's an connection error, since we don't indicate the connection state as a icon in the status bar (yet?). For example: When the user is playing in the configuration screen there's currently no way for them to see if switching between different bridges caused an error. The user needs to manually head back to the main connection screen and verify. Another example is when the user hasn't opened the app at all, but due to upstream connectivity issues the app reaches an error state the user will not notice it.

Adding a notification on errors will be an easy fix and the connection and error state visualization throughout the different screens of the app can be refined as a separate task.