Header can be missing in response

Minor problem which I spent quite some time debugging...

I got the following trace when initializing EWSWrapper:
```
...
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 109, in __init__
    self.version = self.Version(self)
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 118, in __init__
    self.minorbuildnumber = self.get()
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 193, in get
    info = header.find('{%s}ServerVersionInfo' % tns).attrib
AttributeError: 'NoneType' object has no attribute 'find'
```

This was caused by the following response:
```
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
   <s:Body>
      <s:Fault>
         <faultcode xmlns:a="http://schemas.microsoft.com/exchange/services/2006/types">a:ErrorInvalidServerVersion</faultcode>
         <faultstring xml:lang="en-US">The specified server version is invalid.</faultstring>
         <detail>
            <e:ResponseCode xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">ErrorInvalidServerVersion</e:ResponseCode>
            <e:Message xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">The specified server version is invalid.</e:Message>
         </detail>
      </s:Fault>
   </s:Body>
</s:Envelope>
```

With this PR applied, it'll look like this instead, which makes debugging much more easily:
```
...
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 109, in __init__
    self.version = self.Version(self)
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 118, in __init__
    self.minorbuildnumber = self.get()
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 194, in get
    raise Exception('No header in response: %s' % response)
Exception: No header in response: <?xml version="1.0" encoding="utf-8"?><s:Envelope xmlns:s="http://schemas.xmlsoap.org/
soap/envelope/"><s:Body><s:Fault><faultcode xmlns:a="http://schemas.microsoft.com/exchange/services/2006/types">a:ErrorI
nvalidServerVersion</faultcode><faultstring xml:lang="en-US">The specified server version is invalid.</faultstring><deta
il><e:ResponseCode xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">ErrorInvalidServerVersion</e:Res
ponseCode><e:Message xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">The specified server version i
s invalid.</e:Message></detail></s:Fault></s:Body></s:Envelope>
```

Not sure why the server didn't accept the request as I haven't looked into that yet. Will file as a separate issue.