Invalid use of httplib2.error.ServerNotFoundError instead of httplib2.ServerNotFoundError

When upgrading from 1.62 to 1.70 I get a new exception, stepping back to 1.62 makes it work again.

Looks like a simple typo after a quick look.

Full steps to reproduce the issue:
1. Run GYB on a Google Account
2. v1.62 works fine
3. v1.70 fails with exception below

**Simply replacing "httplib2.error.ServerNotFoundError" at line 757 with "httplib2.ServerNotFoundError" makes it work again.**

Expected outcome (what are you trying to do?):
`It works`

Actual outcome (what errors or bad behavior do you see instead?):

```
GYB needs to refresh 27768 messages
Traceback (most recent call last):
File "gyb.py", line 745, in callGAPI
return method.execute()
File "/usr/lib/python3/dist-packages/googleapiclient/_helpers.py", line 130, in positional_wrapper
return wrapped(*args, **kwargs)
File "/usr/lib/python3/dist-packages/googleapiclient/http.py", line 1493, in execute
callback(request_id, response, exception)
File "gyb.py", line 1659, in refresh_message
raise exception
File "/usr/lib/python3/dist-packages/googleapiclient/http.py", line 1487, in execute
raise HttpError(resp, content, uri=request.uri)
googleapiclient.errors.HttpError:

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "gyb.py", line 2761, in
main(sys.argv[1:])
File "gyb.py", line 2120, in main
callGAPI(gbatch, None, soft_errors=True)
File "gyb.py", line 757, in callGAPI
httplib2.error.ServerNotFoundError) as e:
AttributeError: module 'httplib2' has no attribute 'error'
```

Open gyb.py around line 757 and compare the referenced httplib2 exception with the traceback and the working 1.62 behavior. Reproduce the failure by running GYB against a Google Account, then verify that the refresh operation no longer raises the AttributeError and handles the reported server-not-found exception correctly.

Written by the indexing model from the issue text.