An indicator of some failure adjacent or at the point where the object failed would be a good start, and then you might lean on whatever position you have reserved for messaging, either in a temporary message flash wherever is typical of the web app (top or missle), or corners (growl style).

A slight indication at the location of the error can be as simple as a color change on the border or overlaying the container of the drop area.

An indicator of some failure adjacent or at the point where the object failed would be a good start, and then you might lean on whatever position you have reserved for messaging, either in a temporary message flash wherever is typical of the web app (top or middle), or corners (growl style).

A slight indication at the location of the error can be as simple as a color change on the border or overlaying the container of the drop area.