JAV2I12 — export/unexport errors not well specified

Summary: The text describing PortableRemoteObject does not describe the
possible error cases very completely or precisely. 1. Change the paragraph:

It is an error to call exportObject more than once for the same object.

to the paragraph:

It is an error to call exportObject on an object that is already
exported.

2. Change the sentence in the description of toStub:

The argument object must either be a subclass of PortableRemoteObject
or have been previously the target of a call on
PortableRemoteObject.exportObject.

to the sentences:

The argument object must currently be exported, either because it is
a subclass of PortableRemoteObject or by virtue of a previous call
to PortableRemoteObject.exportObject. If the object is not currently
exported, a NoSuchObjectException is thrown.

3. Change the sentence:

The unexportObject method is used a deregister a server object from
the ORB runtimes, allowing the object to become available for
garbage collection.

to the sentences:

The unexportObject method is used a deregister a currently exported
server object from the ORB runtimes, allowing the object to become
available for garbage collection. If the object is not currently
exported, a NoSuchObjectException is thrown.

Closed, accepted