Rework assert to also give quick introspection of what is wrong

Signed-off-by: Christian Parpart <[email protected]>

Author: christianparpart

src/frequenz/sdk/timeseries/_formula_engine/_formula_formatter.py

@@ -244,7 +244,9 @@ def _format(self, postfix_expr: list[FormulaStep]) -> str:
                 case OpenParen():
                     pass  # We gently ignore this one.
 
-        assert len(self._stack) == 1
+        assert (
+            len(self._stack) == 1
+        ), f"The formula {postfix_expr} is not valid. Evaluation stack left-over: {self._stack}"
         return self._stack[0].value
 
     def _format_binary(self, operator: Operator) -> None: