Better error msg if optional dependencies are missing

Author: Archmonger

src/reactpy/executors/asgi/__init__.py

@@ -1,5 +1,10 @@
-from reactpy.executors.asgi.middleware import ReactPyMiddleware
-from reactpy.executors.asgi.pyscript import ReactPyCsr
-from reactpy.executors.asgi.standalone import ReactPy
+try:
+    from reactpy.executors.asgi.middleware import ReactPyMiddleware
+    from reactpy.executors.asgi.pyscript import ReactPyCsr
+    from reactpy.executors.asgi.standalone import ReactPy
 
-__all__ = ["ReactPy", "ReactPyCsr", "ReactPyMiddleware"]
+    __all__ = ["ReactPy", "ReactPyCsr", "ReactPyMiddleware"]
+except ModuleNotFoundError as e:
+    raise ModuleNotFoundError(
+        "ASGI executors require the 'reactpy[asgi]' extra to be installed."
+    ) from e