Throw error when agent cannot be started (elixir-grpc#23)

Update the agent function `start_agent_on_first_call/1` to continue to handle `already_started` and the normal start response, but now crashing on other input
Previously if there's an error when starting an agent, the error is
effectively silenced. I'm guessing this is to suppress `already_started`
"errors". Unfortunately this also suppress any error

With this change, `already_started` will be ignored, but any other error
will blow up, making it easy to see what went wrong. I considered
logging an error and continuing, but it's simpler to blow up, and feels
aligned with "let it crash"

lib/grpc_reflection/service/agent.ex

@@ -63,8 +63,9 @@ defmodule GrpcReflection.Service.Agent do
 
   defp start_agent_on_first_call({name, services}) do
     # lazy start agent on call
-    if match?({:ok, _}, GrpcReflection.DynamicSupervisor.start_child(name, services)) do
-      Logger.info("Started reflection agent #{name}")
+    case GrpcReflection.DynamicSupervisor.start_child(name, services) do
+      {:ok, _} -> Logger.info("Started reflection agent #{name}")
+      {:error, {:already_started, _}} -> :ok
     end
 
     name