Add better error message

Author: ChrisJefferson

PackageInfo.g

@@ -15,8 +15,8 @@ SetPackageInfo( rec(
 
 PackageName := "BacktrackKit",
 Subtitle := "An Extensible, easy to understand backtracking framework",
-Version := "0.3.1",
-Date := "07/11/2019", # dd/mm/yyyy format
+Version := "0.4",
+Date := "05/05/2021", # dd/mm/yyyy format
 
 
 Persons := [

gap/BacktrackKit.gi

@@ -66,7 +66,7 @@ InstallMethod(ApplyFilters, [IsBTKitState, IsTracer, IsObject],
             return false;
         fi;
     else
-        ErrorNoReturn("Invalid filter?");
+        ErrorNoReturn("BacktrackKit refiners must be 'fail' or a single function");
     fi;
     return true;
 end);