Don't doubly wrap TransportException in smart HTTP client

Don't doubly wrap TransportException in smart HTTP client

If the readAdvertisedRefs() method throws an exception, its already
closed the connection and wrapped the underlying cause inside of a
suitable TransportException object that it is throwing.  We shouldn't
catch IOException and rethrow a wrapped copy here, because we'll double
wrap the exception thrown by readAdvertisedRefs.  This may obsecure the
root cause of the connection failure from the end-user.

Change-Id: I0ca61560f9888c666323dac8a5582aab25e897ff
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>