improve baseline warning message (#2650)

Author: znxftw

src/BenchmarkDotNet/Analysers/BaselineCustomAnalyzer.cs

@@ -31,7 +31,8 @@ protected override IEnumerable<Conclusion> AnalyseSummary(Summary summary)
                     continue;
 
                 var message = "A question mark '?' symbol indicates that it was not possible to compute the " +
-                                $"({columnNames}) column(s) because the baseline value is too close to zero.";
+                                $"({columnNames}) column(s) because the baseline or benchmark could not be found, or " +
+                                $"the baseline value is too close to zero.";
 
                 yield return Conclusion.CreateWarning(Id, message);
             }