Repository: jupyterhub/jupyterhub

Issue:
Maybe a bug about module checking
### Bug description
<!-- Use this section to clearly and concisely describe the bug. -->
If I use conda to install only jupyterhub and python (conda install -c conda-forge python=3.9 jupyterhub), the following message showed as someone try to login:

```
Failed to set groups [Errno 1] Operation not permitted
Traceback (most recent call last):
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/bin/jupyterhub-singleuser", line 7, in <module>
    from jupyterhub.singleuser import main
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/lib/python3.9/site-packages/jupyterhub/singleuser/__init__.py", line 5, in <module>
    from .app import main
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/lib/python3.9/site-packages/jupyterhub/singleuser/app.py", line 38, in <module>
    raise _import_error
TypeError: exceptions must derive from BaseException
```
I think the problem is the lines from 32 to 36 in jupyterhub/singleuser/app.py
```
        except ImportError as e:
            continue
            if _import_error is None:
                _import_error = e
        else:
            break
```

I changed that with:
```
        except ImportError as e:
            if _import_error is None:
                 _import_error = e
            else:
                break
            continue
```
then the better message showed:
```
Failed to set groups [Errno 1] Operation not permitted
Traceback (most recent call last):
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/bin/jupyterhub-singleuser", line 7, in <module>
    from jupyterhub.singleuser import main
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/lib/python3.9/site-packages/jupyterhub/singleuser/__init__.py", line 5, in <module>
    from .app import main
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/lib/python3.9/site-packages/jupyterhub/singleuser/app.py", line 38, in <module>
    raise _import_error
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/lib/python3.9/site-packages/jupyterhub/singleuser/app.py", line 30, in <module>
    App = import_item(JUPYTERHUB_SINGLEUSER_APP)
  File "/home/someone/bin/anaconda3/envs/py39jupyterhub222/lib/python3.9/site-packages/traitlets/utils/importstring.py", line 30, in import_item
    module = __import__(package, fromlist=[obj])
ModuleNotFoundError: No module named 'jupyter_server'
```
The above message let me know that I have to install jupyter_server.
This issue can be closed anytime.
Any suggestion is welcome.



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Resolving patch:
diff --git a/jupyterhub/singleuser/app.py b/jupyterhub/singleuser/app.py
--- a/jupyterhub/singleuser/app.py
+++ b/jupyterhub/singleuser/app.py
@@ -29,9 +29,9 @@
         try:
             App = import_item(JUPYTERHUB_SINGLEUSER_APP)
         except ImportError as e:
-            continue
             if _import_error is None:
                 _import_error = e
+            continue
         else:
             break
     if App is None:


Test patch: