Fix 503 on account/container HEAD w/invalid format.

A HEAD request to /v1/a[/c]?format=%FF would result in a 503 since
there was an unhandled UnicodeDecodeError. Now it doesn't.

bug 1190395

Change-Id: I4d8ec677092617391fe154a25f22d919536a72a5
Merge "Fix 503 on account/container HEAD w/invalid format."