add test with failing conversion

tests/test_pyfunction.rs

@@ -113,7 +113,7 @@ fn test_functions_with_function_args() {
 
 #[cfg(not(Py_LIMITED_API))]
 fn datetime_to_timestamp(dt: &PyAny) -> PyResult<i64> {
-    let dt: &PyDateTime = dt.extract()?;
+    let dt: &PyDateTime = dt.extract();
     let ts: f64 = dt.call_method0("timestamp")?.extract()?;
 
     Ok(ts as i64)
@@ -147,6 +147,23 @@ fn test_function_with_custom_conversion() {
     )
 }
 
+#[cfg(not(Py_LIMITED_API))]
+#[test]
+fn test_function_with_custom_conversion_error() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+
+    let custom_conv_func = wrap_pyfunction!(function_with_custom_conversion)(py).unwrap();
+
+    py_expect_exception!(
+        py,
+        custom_conv_func,
+        "custom_conv_func(['a'])",
+        PyTypeError,
+        "argument 'timestamp': 'list' object cannot be converted to 'PyDateTime'"
+    );
+}
+
 #[test]
 fn test_raw_function() {
     let gil = Python::acquire_gil();