Do not show an error message when draging and dropping text

When the browser reports a drag of items other than files (for example,
text) and then triggers a drop event with no files no error message
should be shown to the user, as in that case there would be no highlight
of the drop zone and no indication that the drop would be valid (except
for the mouse cursor); the error message should be shown only when
the drop event with no files follows a file drag.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>