Update sphinx_needs/filter_common.py

Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>

sphinx_needs/filter_common.py

@@ -344,9 +344,8 @@ def filter_single_need(
         else:
             result = eval(filter_string, filter_context)
         if not isinstance(result, bool):
-            # Raises NeedsInvalidFilter if the result is a string type
             raise NeedsInvalidFilter(
-                f"Error when evaluating filter: expected output to have True/False but got a string <{result}>"
+                f"Filter did not evaluate to a boolean, instead {type(result)}: {result}"
             )
     except Exception as e:
         raise NeedsInvalidFilter(f"Filter {filter_string} not valid. Error: {e}.")