Fix play assertion error

[Markus28]

This PR addresses #126
I just replaced the error message with an actual exception.
I guess logger.error can be a somewhat confusing name for a function that only does logging. Searching through the repo,

Gymnasium/gymnasium/utils/save_video.py

Line 86 in 5674a52

logger.error(f"Expected a list of frames, got a {type(frames)} instead.")

seems to have the same problem.

[pseudo-rnd-thoughts]

Can we have a short test that this works as expected?

[axb2035]

@pseudo-rnd-thoughts
Hi. For 'short test' do you mean 1. adding an assert before the conditional i.e.

assert env.render_mode is not None
if env.render_mode not in {"rgb_array", "rgb_array_list"}:

or 2. attaching a script to the PR like:

import gymnasium as gym
from gymnasium.utils.play import play

mapping = {"2": 1,  # Down.
           "4": 0,  # Left.
           "6": 2,  # Right.
           "8": 3,  # Up.
           }

# Break by passing non-supported render_mode.
try:
    play(gym.make('FrozenLake-v1', render_mode='human'), keys_to_action=mapping)
except Exception as e:
    print("Unsupported render_mode test:", str(e))

# Confirm by passing supported render_mode.
try:
    play(gym.make('FrozenLake-v1', render_mode='rgb_array'), keys_to_action=mapping)
    print("No exceptions.")
except Exception as e:
    print("Unexpected exception:", str(e))

[axb2035]

BTW: logger does send a error to the console as well so it is providing some feedback on the error to the user running it. Also, if for some reason you were running it remotely you might not have a console, and want to see the output of logs if it fails. Though I appreciate this is probably an extreme edge case.

[pseudo-rnd-thoughts]

In tests we have a large collection of tests for the whole project using pytest.
In tests/utils/test_play.py, add option 2 in there