Improve error message for auto_container* fixtures

If the user does not add the `auto_container_parametrize` call to
`pytest_generate_tests()` in `conftest.py`, then they get a weird error
message. This commit improves the error message, indicating where the issue
might be.
This should make it easier to spot issues like
#179 easier.

pytest_container/plugin.py

@@ -76,7 +76,13 @@ def fixture(
         pytest_generate_tests.
         """
 
-        container, _ = container_and_marks_from_pytest_param(request.param)
+        try:
+            container, _ = container_and_marks_from_pytest_param(request.param)
+        except AttributeError as attr_err:
+            raise RuntimeError(
+                "This fixture was not parametrized correctly, "
+                "did you forget to call `auto_container_parametrize` in `pytest_generate_tests`?"
+            ) from attr_err
         _logger.debug("Requesting the container %s", str(container))
 
         if scope == "session" and container.singleton: