webdav: log a non-nil error for unsupported HTTP methods.

Change-Id: Idcd7c225efda94021ba18ca33f3fe7e8565140fe
Reviewed-on: https://go-review.googlesource.com/3861
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Nick Cooper <nmvc@google.com>