Add warnings that browser doesn't work without internet

[larrysalibra]

When a user's internet connection isn't working or is unreliable: tethering, traveling, wifi issue etc,

The browser fails in random ways without any actionable notice to users.

[jackzampolin]

This would be really nice to add. I think that to ship this warning we need a better path for users to connect to a local blockstack-core instance.