Fix ART failure in PhotonVertexSelectionTool
During the approval discussion of !26774 (merged), some WARNING messages have been changed to ERROR. This MR reverts some of these changes. The reason is the StatusCode::Failure in getVertex function are handled in the overridden getVertex function, accessible from the interface. Therefore returning a StatusCode::Failure with a WARNING should not be a problem. The problem was found during ART test, since the function returns Success but with an ERROR message
Meanwhile, I've reduced the verbosity of the tool, mainly when run to produce unskimmed sample