log.error should log failures, not strings.

In the default_errback for the routing dispatcher in Vumi we do:
```python
    def default_errback(self, f, msg, connector_name):
        log.error("Error routing message for %s" % (connector_name,))
        log.error(f)
```
but Twisted docs (http://twistedmatrix.com/documents/10.1.0/web/howto/web-in-60/logging-errors.html) say we should be doing something more like:
```python
    def default_errback(self, f, msg, connector_name):
        log.error(f, "Error routing message for %s" % (connector_name,))
```
There may be other places we are doing this too.