utils: Improve the wording of the error message for duplicate accounts
utils: Improve the wording of the error message for duplicate accounts

felipeborges/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts,Distrotech/gnome-online-accounts,halfline/gnome-online-accounts,dnohales/gnome-online-accounts,Distrotech/gnome-online-accounts,ritzk/gnome-online-accounts,felipeborges/gnome-online-accounts,ritzk/gnome-online-accounts,dnohales/gnome-online-accounts,halfline/gnome-online-accounts,felipeborges/gnome-online-accounts