Fix handling of bad paths in Swift middleware

The path //v1/account does not denote any Swift resource. More
importantly, if you call swift.common.utils.split_path() on it (as the
Swift middleware does), then you get a ValueError.

Since it's a bad path, it doesn't refer to any Swift resource, so
there's no stats to publish, so we can just catch and ignore
ValueError.

Change-Id: I9908beef8478c5407985435c1526a9c72edb1d3a
Merge "Fix handling of bad paths in Swift middleware"