Expand DataTransferList remove exception message.

This occurs when an attempt is made to remove an item from the list
while it is not writable. Removal is permitted only during certain
events (e.g. dragstart, copy).

BUG=270033

Review URL: https://codereview.chromium.org/110333003

git-svn-id: svn://svn.chromium.org/blink/trunk@163898 bbb929c8-8fbe-4397-9dbb-9b2b20218538