need to delete and re-enter locations after forecast menu item crash fix

[Felix Lawrence]

While bug #739420 (et al.) has been fixed, the problem occurred when locations were created. So the fix for #739420 won't help people who created their locations before the bug was fixed, unless they delete their locations and re-enter them. We need to do either:
1) Somehow automatically update only the affected locations, and ideally only run this once per computer, or
2) Catch the exception and instead of crashing, either update the affected locations or tell the user to delete and re-enter the location.

We should also explicitly check that Google doesn't return empty weather info.

[Vadim Rutkovsky (roignac)]

I don't think we should add the code, which will convert incorrect locations - as it would result in unnecessary server requests and parts of code, which will be not safe to remove.
Second option would better - a simple message box on Preferences dialog would be enough

[Vadim Rutkovsky (roignac)]

As foggy version now uses dconf to store settings, I don't think we should migrate user data (due to the bug with incorrectly stored coordinates) - this will cause way too much desktopcouch dependencies