Handle corrupted scope values

Due to a bug (fixed some commits ago) in the UsersController of the
settings app the scope of the properties can be null (for example, if
lookup server upload was disabled and the user then changed the display
name in the profile information). In that case now the scope menu icon
shows an error to inform the user.

The scope value will not change when other properties are modified until
the user chooses an explicit value from the menu. Note that until a
scope is explicitly set the property will behave as if it is private.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>