desktop manager service: fix and improve default error

Before the error if the wrong default desktop was chosen would be:

/nixpkgs-channels/lib/modules.nix:282:11:
Default desktop manager ($(defaultDM)) not found.

which has the string interpolation done incorreclty. Now that is fixed
and it is more user-friendly as:

/nixpkgs-channels/lib/modules.nix:282:11:

Default desktop manager (gnome) not found.
Probably you want to change
  services.xserver.desktopManager.default = "gnome";
to one of
  services.xserver.desktopManager.default = "gnome3";
  services.xserver.desktopManager.default = "none";