Prevent exception when pool is not started, return error tuple instead

[guisehn]

If the pool process is not started, prevent the exception when calling Pool.get_channel():

** (ArgumentError) unknown registry: MyApplication.Pool.Registry
    (elixir 1.18.1) lib/registry.ex:1457: Registry.key_info!/1
    (elixir 1.18.1) lib/registry.ex:590: Registry.lookup/2
    (conn_grpc 0.2.1) lib/conn_grpc/pool.ex:132: ConnGRPC.Pool.get_channel/1
    iex:1: (file)

Instead, return {:error, :not_started}

[orca-security-us]

Orca Security Scan Summary

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