<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_agent_class(alg):
 """Returns the class of a known agent given its name."""


    try:
        return _get_agent_class(alg)
    except ImportError:
        from ray.rllib.agents.mock import _agent_import_failed
        return _agent_import_failed(traceback.format_exc())