Inform user if modem isn't available for creating conversation

When chatty is launched from other applications, chatty silently fails if no appropriate account is available.

Say for example, if the user clicks a phone number from gnome-contacts, chatty is launched. But if no supported modem is available, chatty fails silently and the user shall have no clue what happened. May be an error dialog or in-app notification be shown?

Related: #296 (closed)

/cc @tobias.bernard