Add context to AssertionError for derived metric expr (#1231)

Customers have been hitting this assertion error with no context. Likely
this error was added with the assumption that validations would prevent
this case, but apparently that is not true. I'll also be following up on
the validations to make sure that gets handled in the future.

metricflow/plan_conversion/dataflow_to_sql.py

@@ -677,7 +677,9 @@ def visit_compute_metrics_node(self, node: ComputeMetricsNode) -> SqlDataSet:
                     column_name=expr, input_measure=input_measure, from_data_set_alias=from_data_set_alias
                 )
             elif metric.type is MetricType.DERIVED:
-                assert metric.type_params.expr
+                assert (
+                    metric.type_params.expr
+                ), "Derived metrics are required to have an `expr` in their YAML definition."
                 metric_expr = SqlStringExpression(sql_expr=metric.type_params.expr)
             elif metric.type == MetricType.CONVERSION:
                 conversion_type_params = metric.type_params.conversion_type_params