Fixed bug raising AttributeError instead of a descriptive error of the actual problem

Sample soap response which triggered this:
```xml
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
   <s:Header>
      <h:ServerVersionInfo MajorVersion="14" MinorVersion="3" MajorBuildNumber="158" MinorBuildNumber="1" Version="Exchange2010_SP2" xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types" xmlns="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"/>
   </s:Header>
   <s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
      <m:GetItemResponse xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
         <m:ResponseMessages>
            <!--
            <m:GetItemResponseMessage>
            ...
            </m:GetItemResponseMessage>
            -->
            <m:GetItemResponseMessage ResponseClass="Error">
               <m:MessageText>An internal server error occurred. Try again later.</m:MessageText>
               <m:ResponseCode>ErrorInternalServerTransientError</m:ResponseCode>
               <m:DescriptiveLinkKey>0</m:DescriptiveLinkKey>
               <m:Items/>
            </m:GetItemResponseMessage>
            <!--
            <m:GetItemResponseMessage>
            ...
            </m:GetItemResponseMessage>
            -->
         </m:ResponseMessages>
      </m:GetItemResponse>
   </s:Body>
</s:Envelope>
```

Original stack trace
```
...
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 432, in listCalendarEvent
    return self.listItems(type, id, start, end, on_behalf, shape, categories, folder_id=folder_id)
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 1330, in listItems
    extended_items = self.listItems(type="CALENDAR", id=ids, additional=additional)
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 1149, in listItems
    (rspcode, msg.MessageText))
AttributeError: 'list' object has no attribute 'MessageText'
```

Stack trace after fix applied:
```
Traceback (most recent call last):
...
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 432, in listCalendarEvent
    return self.listItems(type, id, start, end, on_behalf, shape, categories, folder_id=folder_id)
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 1330, in listItems
    extended_items = self.listItems(type="CALENDAR", id=ids, additional=additional)
  File "C:\Python27\lib\site-packages\EWSWrapper_py\EWSWrapper.py", line 1149, in listItems
    (rspcode, el.MessageText))
Exception: Error code: ErrorInternalServerTransientError message: An internal server error occurred. Try again later.
```