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>