informative error on missing dependencies for singleuser server

- defer jupyter_core import that caused earlier, less informative ImportError
- point to `pip install jupyterhub[singleuser]` in the error
- use `raise from` so original import error is still reported

jupyterhub/singleuser/__init__.py

@@ -63,9 +63,29 @@ if _as_extension:
             f"Cannot use JUPYTERHUB_SINGLEUSER_EXTENSION={_extension_env} with JUPYTERHUB_SINGLEUSER_APP={_app_env}."
             " Please pick one or the other."
         )
+    try:
         from .extension import main
+    except ImportError as e:
+        # raise from to preserve original import error
+        raise ImportError(
+            "Failed to import JupyterHub singleuser extension."
+            " Make sure to install dependencies for your single-user server, e.g.\n"
+            "    pip install 'jupyterhub[singleuser]'"
+        ) from e
 else:
+    try:
         from .app import SingleUserNotebookApp, main
+    except ImportError as e:
+        # raise from to preserve original import error
+        if _app_env:
+            _app_env_log = f"JUPYTERHUB_SINGLEUSER_APP={_app_env}"
+        else:
+            _app_env_log = "default single-user server"
+        raise ImportError(
+            f"Failed to import {_app_env_log}."
+            " Make sure to install dependencies for your single-user server, e.g.\n"
+            "    pip install 'jupyterhub[singleuser]'"
+        ) from e
 
     # backward-compatibility
     if SingleUserNotebookApp is not None:

jupyterhub/singleuser/_disable_user_config.py

@@ -22,8 +22,6 @@ rather than keeing these monkey patches around.
 import os
 from pathlib import Path
 
-from jupyter_core import paths
-
 
 def _is_relative_to(path, prefix):
     """
@@ -68,6 +66,10 @@ def _disable_user_config(serverapp):
     2. Search paths for extensions, etc.
     3. import path
     """
+    # delayed import to avoid triggering early ImportError
+    # with unmet dependencies
+    from jupyter_core import paths
+
     original_jupyter_path = paths.jupyter_path()
     jupyter_path_without_home = list(_exclude_home(original_jupyter_path))