correctly handle valid errors in syn join when starting children

the previous commit introduced registry error handling when (re)starting
children. However a child starting and then immediately terminating
without errors (which is a valid use case) would be treated as an error

fix the problem by handling the not_alive error reason in syn.join()

Author: flaviogrossi

lib/syn_supervisor.ex

@@ -944,6 +944,9 @@ defmodule SynSupervisor do
           :ok ->
             {:reply, reply, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}
 
+          {:error, :not_alive} ->
+            {:reply, reply, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}
+
           {:error, reason} ->
             Process.exit(pid, :kill)
             {:reply, {:error, {:child_join_failed, reason}}, state}
@@ -954,6 +957,9 @@ defmodule SynSupervisor do
           :ok ->
             {:reply, reply, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}
 
+          {:error, :not_alive} ->
+            {:reply, reply, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}
+
           {:error, reason} ->
             Process.exit(pid, :kill)
             {:reply, {:error, {:child_join_failed, reason}}, state}
@@ -1398,6 +1404,10 @@ defmodule SynSupervisor do
         state = delete_child(current_pid, state)
         {:ok, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}
 
+      {:error, :not_alive} ->
+        state = delete_child(current_pid, state)
+        {:ok, save_child(pid, id, mfa, restart, shutdown, type, modules, state)}
+
       {:error, reason} ->
         Process.exit(pid, :kill)