Improve error message if we can't boot epmd, closes #196 (#197)

lib/livebook/application.ex

@@ -39,16 +39,39 @@ defmodule Livebook.Application do
 
   defp ensure_distribution!() do
     unless Node.alive?() do
-      System.cmd("epmd", ["-daemon"])
+      case System.cmd("epmd", ["-daemon"]) do
+        {_, 0} ->
+          :ok
+
+        _ ->
+          abort!("""
+          could not start epmd (Erlang Port Mapper Driver). Livebook uses epmd to \
+          talk to different runtimes. You may have to start epmd explicitly by calling:
+
+              epmd -daemon
+
+          Or by calling:
+
+              elixir --sname test -e "IO.puts node()"
+
+          Then you can try booting Livebook again
+          """)
+      end
+
       {type, name} = get_node_type_and_name()
 
       case Node.start(name, type) do
         {:ok, _} -> :ok
-        {:error, _} -> raise "failed to start distributed node"
+        {:error, reason} -> abort!("could not start distributed node: #{inspect(reason)}")
       end
     end
   end
 
+  defp abort!(message) do
+    IO.puts("\nERROR!!! [Livebook] " <> message)
+    System.halt(1)
+  end
+
   defp get_node_type_and_name() do
     Application.get_env(:livebook, :node) || {:shortnames, random_short_name()}
   end

lib/livebook/runtime/elixir_standalone.ex

@@ -55,7 +55,9 @@ defmodule Livebook.Runtime.ElixirStandalone do
   defp start_elixir_node(elixir_path, node_name, eval) do
     # Here we create a port to start the system process in a non-blocking way.
     Port.open({:spawn_executable, elixir_path}, [
+      :binary,
       :nouse_stdio,
+      :hide,
       args: elixir_flags(node_name) ++ ["--eval", eval]
     ])
   end

lib/livebook/runtime/mix_standalone.ex

@@ -91,6 +91,7 @@ defmodule Livebook.Runtime.MixStandalone do
     Port.open({:spawn_executable, elixir_path}, [
       :binary,
       :stderr_to_stdout,
+      :hide,
       args: elixir_flags(node_name) ++ ["-S", "mix", "run", "--eval", eval],
       cd: project_path
     ])