dnd: Skip drag target when its acceptDrop() throws an exception

dnd: Skip drag target when its acceptDrop() throws an exception

In the case of bugs in a drag target's acceptDrop() function, it may
throw an exception. In the previous code, this would break out of the
loop entirely and never cancel the drag, so the mouse button release
event would be ignored and you would have to press Esc to get out of the
drag.

In this change, if acceptDrop() throws an exception, we log it and move
on to the next parent target instead.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/777