pass exc_info along when logging exs, dont split and log tb separately

ends up looking like this now as single log msg:

```
2014-09-30 07:03:06,891 : ERROR : paramiko.transport : 16813 : Thread-2 : Exception: Error reading SSH protocol banner
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/paramiko/transport.py", line 1582, in run
    self._check_banner()
  File "/usr/local/lib/python2.7/dist-packages/paramiko/transport.py", line 1707, in _check_banner
    raise SSHException('Error reading SSH protocol banner' + str(e))
SSHException: Error reading SSH protocol banner
```

closes #405