Undefined name 'BadProxyCommand'

paramiko/proxy.py refers to an undefined exception name `BadProxyCommand` twice.

I checked with pyflakes to see if there might be any other similar problems, and found none:
```
mg@platonas: ~/src/paramiko [git:master=] $ pyflakes paramiko|grep -v 'imported but unused'|grep -v 'never used'|grep -v 'unable to detect'
paramiko/proxy.py:67: undefined name 'BadProxyCommand'
paramiko/proxy.py:83: undefined name 'BadProxyCommand'
```