Add AbstractUnsafe.annotateConnectException()

Motivation:

JDK's exception messages triggered by a connection attempt failure do
not contain the related remote address in its message.  We currently
append the remote address to ConnectException's message, but I found
that we need to cover more exception types such as SocketException.

Modifications:

- Add AbstractUnsafe.annotateConnectException() to de-duplicate the
  code that appends the remote address

Result:

- Less duplication
- A transport implementor can annotate connection attempt failure
  message more easily