Throw StreamUnusableException after close/detach
This commit adjusts the behavior of close and detach to set the body
to null rather than an empty string. All methods that support throwing
an exception will now throw StreamUnusableException to indicate the
stream has been closed and is thus unusable.
The methods that do not have "@throws RuntimeException" in their
docblocks usually return the zero value of their type. The sole
exception is the eof (End of File) function, since both true and false
is wrong; there's no stream to be at the end of. In this case, true is
returned because that's how the existing implementation works; pointer
is at payload length.
StreamUnusableException is a kind of IllegalOperationException, a new
exception that tells upstream callers which function call failed and
why. ReadOnlyStringStream now throws IllegalOperationException when a
user calls the write() method.