Allow checkers to read all OWNERS files

* Fix problem of some submit checkers that might submit a change
  without full owner info/approval.
* GetOwners API and UI Action still obey the read permission.
* Log more messages when a project or file cannot be read.
* Check and log errors when owner preferred email is null.

Change-Id: Id3d39fd11f8f24db8ff703d803dffbf6bcbfa2d7