fix: error out early if ray cluster does not have resources

[parthchadha]

What does this PR do ?

Throw error early if resources are not available.

ValueError: Not enough GPUs available. Requested 16 GPUs, but only 8 are available in the cluster.
Traceback (most recent call last):
  File "/core/gh/NeMo-Reinforcer/./examples/run_grpo_math.py", line 293, in <module>
    main()
  File "/core/gh/NeMo-Reinforcer/./examples/run_grpo_math.py", line 275, in main
    ) = setup(config, tokenizer, dataset, val_dataset)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/core/gh/NeMo-Reinforcer/nemo_reinforcer/algorithms/grpo.py", line 205, in setup
    cluster = RayVirtualCluster(
              ^^^^^^^^^^^^^^^^^^
  File "/core/gh/NeMo-Reinforcer/nemo_reinforcer/distributed/virtual_cluster.py", line 162, in __init__
    self._init_placement_groups(placement_group_strategy)
  File "/core/gh/NeMo-Reinforcer/nemo_reinforcer/distributed/virtual_cluster.py", line 189, in _init_placement_groups
    raise ValueError(
ValueError: Not enough GPUs available. Requested 16 GPUs, but only 8 are available in the cluster.

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