Usecase:
Suppose you implement the MoveOperation interface
for java files. Before the move is done the
operation can be canceled in two points canMove()
and beforeMove(). Both can return false. A reason
can be that eg a filename is not valid classname.
In that case a client calling ProjectMember.move
has no notion about the reason of the
cancellation. How do they ever detect that
something is wrong and what is the reason?

Current situation:
If the PM finds out an operation cannot be
executed (eg canMove == false) it throws an
IOException("Operation is not permitted").

Proposal:
There should be some way in the api/spi to let the
client know why the operation is not permitted. It
is insufficient to return just true or false.
Otherwise all tests before operations would have
to perform each client by oneself thus tests in
operation handlers would have no sense.