Avoid IOStream.close(exc_info=True)

Avoid IOStream.close(exc_info=True)

close(exc_info=True) calls sys.exc_info() to get the "current" exception.
Unfortunately, on Python 2 this is the last raised exception even if it
was caught and silenced (by contrast with Python 3, which has lexically
nested exceptions).  This could set ``IOStream.error`` and therefore
``TCPClient.connect``'s raised exception to the wrong error.

Fix by passing the explicit error instance instead.