prepare Popen for OSError exception

Hi,

If xvfb is not installed, and running the tests, the following output is generated:

```python
======================================================================
ERROR: testGoogleHomepage (__main__.TestPages)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "xx.py", line 12, in setUp
    self.xvfb.start()
  File "/home/danclaudiupop/envs/account/local/lib/python2.7/site-packages/xvfbwrapper.py", line 55, in start
    stderr=open(os.devnull),
  File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1259, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory
```

I prepared the app for OSError exception and provided a more friendly error message.  Also, this makes it so we can run the tests even if we didn't start the tests from an X session.

```
testGoogleHomepage (__main__.TestPages) ... Failed to run Xvfb :1095 -screen 0 1280x720x24
ok
```