Ensure Retry methods returns only one error and that is correctly handled

In fixing MB-28667, we chose to add a new return value to handle timeout during retry as we didn't want to change all error handling paths as it was too big a change at that point in the release. This bug is to ensure:

(a) common.Timeout is changed to type "error" and is returned as a normal error, i.e.:
CallbackFunc func(arg ...interface{}) error in util.go

(b) Every call to util.Retry must either handle the returned error, or propogate it up to parent. If an error must be ignored, then we should log it at some suitable log level – so it is clear we are ignoring it explicitly.