Add a handleDbException() method to BaseActivity

This adds a handleDbException() method to BaseActivity and a corresponding method for fragments that calls through to the activity. For now, the method just finishes the activity and NavDrawerActivity overrides it to do nothing, and all the error places marked with TODO that finish the activity call the method instead.

That gives us zero functional improvement over the status quo, but it allows us to change the default behaviour easily, and then we can start thinking about which cases should have non-default behaviour.

First part of #469