Log all exceptions

Continuation from https://github.com/graphite-project/graphite-web/pull/936
Also here it'd be more useful to return a message. `log.exception()` automatically adds `sys.exc_info` to the log record so the traceback is already there.

Perhaps something like

```python
log.exception('{0} {1}'.format(request.method, request.build_absolute_uri()))
```