Show an error when an unexpected version of Pythonx is installed

lib/livebook/runtime/definitions.ex

@@ -508,4 +508,6 @@ defmodule Livebook.Runtime.Definitions do
   def pythonx_dependency() do
     %{dep: {:pythonx, "~> 0.4.0"}, config: []}
   end
+
+  def pythonx_requirement(), do: "~> 0.4.0"
 end

lib/livebook/runtime/evaluator.ex

@@ -1053,9 +1053,10 @@ defmodule Livebook.Runtime.Evaluator do
   end
 
   defp ensure_pythonx() do
-    if Code.ensure_loaded?(Pythonx) do
-      :ok
-    else
+    pythonx_requirement = Livebook.Runtime.Definitions.pythonx_requirement()
+
+    cond do
+      not Code.ensure_loaded?(Pythonx) ->
         message =
           """
           Pythonx is missing, make sure to add it as a dependency:
@@ -1065,9 +1066,22 @@ defmodule Livebook.Runtime.Evaluator do
 
         exception = RuntimeError.exception(message)
         {{:error, :error, exception, []}, []}
+
+      not Version.match?(pythonx_version(), pythonx_requirement) ->
+        message =
+          "this Livebook version requires Pythonx #{pythonx_requirement}," <>
+            " but #{pythonx_version()} is installed, please update the dependency"
+
+        exception = RuntimeError.exception(message)
+        {{:error, :error, exception, []}, []}
+
+      true ->
+        :ok
     end
   end
 
+  defp pythonx_version(), do: List.to_string(Application.spec(:pythonx)[:vsn])
+
   defp identifier_dependencies(context, tracer_info, prev_context) do
     identifiers_used = MapSet.new()
     identifiers_defined = %{}

lib/livebook/runtime/evaluator/formatter.ex

@@ -85,15 +85,6 @@ defmodule Livebook.Runtime.Evaluator.Formatter do
     %{type: :terminal_text, text: "Ok", chunk: false}
   end
 
-  def format_result(:"pyproject.toml", {:error, _kind, _error, _stacktrace}) do
-    formatted =
-      "Error, see the output above for details"
-      |> error_color()
-      |> IO.iodata_to_binary()
-
-    %{type: :error, message: formatted, context: nil}
-  end
-
   def format_result(_language, {:error, kind, error, stacktrace}) do
     formatted = format_error(kind, error, stacktrace)
     %{type: :error, message: formatted, context: error_context(error)}