This fixes PrintTargetWindows::BeginPrinting to detect when the user cancels and have it return NS_ERROR_ABORT in that case. The rest of the changes are simply making sure that the various call points up the call stack don't print a warning message if NS_ERROR_ABORT is returned up from PrintTargetWindows::BeginPrinting. MozReview-Commit-ID: 6xZ5SPje6TT