adding more details to AsgElectronLikelihoodTool initialization failure error messages

In order to investigate ATLASRECTS-6418: adding as suggested by @christos a bit more information (paths to the specified and resolved config and PDF files) to the error messages printed by the AsgElectronLikelihoodTool when initialization fails. Also including a couple trivial checks and printing warnings accordingly (e.g. when the default PDF file name is used, which should not really happen). The tool's behaviour should be the same as before with one exception: the initialization will fail early if the PDF filename is empty (but previously that would probably have caused a failure soon afterwards anyway). In principle, for normal usage there should be no more printouts in the log than currently.