Datastore fetch needs to encapsulate all gRPC errors

[theacodes]

This code:

    try:
        start_date = datetime.datetime(1990, 1, 1)
        query = client.query(kind='Task')
        query.add_filter(
            'created', '>', start_date)
        query.add_filter(
            'priority', '>', 3)

        return list(query.fetch())

    except google.cloud.exceptions.BadRequest:
        pass

Raises a grpc._channel._Rendezvous exception instead of an expected subclass of google.cloud.exceptions.BadRequest (I'd also accept google.cloud.exceptions.GoogleCloudError):

grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with (StatusCode.INVALID_ARGUMENT, Cannot have inequality filters on multiple properties: [created, priority])>

[dhermes]

@jonparrott The title of this issue is misleading. We wrap in some places, but have just done it on an ad hoc basis.

[theacodes]

@dhermes feel free to rename it as needed.