AnalysisTop: replace error by warning for exclusion of named btag uncertainties - ANALYSISTO-1333

As described in the ticket, this MR replaces (twice) an error by a warning, when the AnalysisTop user tried to extract a systematic uncertainty from the eigenvector decomposition for the btagging uncertainties, when no systematic with the specified name exists for one of the requested working point. The current situation prevents the user from using several working point if one of them doesn't have the systematic in question. A use-case is the Continuous "working point" which often has different systematics.

NB: this feature could/should also be added for main, but I don't think the cherry-picking would work out of the box. So I made this MR for main: !64446 (merged)