Raise errors on individual problematic extensions when listing extension

[Zsailer]

Fixes #1134. We can backport this to 1.x once it's merged.

This no longer uses an ExtensionManager to load all the extensions as a bunch. Instead, it loads each extension separately, adds a try/catch clause for each individual extension validation, logs the error (if found) for each extension, and doesn't block the validation for other extensions.

Here is an example of the new output:

Config dir: /Users/zsailer/.jupyter

Config dir: /Users/env/etc/jupyter
    jupyter_server_terminals enabled
    - Validating jupyter_server_terminals...
       X Jupyter Server Terminals requires Jupyter Server 2.0+
    Traceback (most recent call last):
      File "/Users/zsailer/Documents/Work/oss/internal/jupyter_server/jupyter_server/extension/serverextension.py", line 336, in list_server_extensions
        extension = ExtensionPackage(name=name, enabled=enabled)
      File "/Users/zsailer/Documents/Work/oss/internal/jupyter_server/jupyter_server/extension/manager.py", line 166, in __init__
        super().__init__(*args, **kwargs)
      File "/Users/env/lib/python3.8/site-packages/traitlets/traitlets.py", line 1238, in __init__
        super_kwargs[key] = value
      File "/Users/env/lib/python3.8/contextlib.py", line 120, in __exit__
        next(self.gen)
      File "/Users/env/lib/python3.8/site-packages/traitlets/traitlets.py", line 1348, in hold_trait_notifications
        value = trait._cross_validate(self, getattr(self, name))
      File "/Users/env/lib/python3.8/site-packages/traitlets/traitlets.py", line 729, in _cross_validate
        value = obj._trait_validators[self.name](obj, proposal)
      File "/Users/env/lib/python3.8/site-packages/traitlets/traitlets.py", line 1132, in __call__
        return self.func(*args, **kwargs)
      File "/Users/zsailer/Documents/Work/oss/internal/jupyter_server/jupyter_server/extension/manager.py", line 175, in _validate_name
        self._module, self._metadata = get_metadata(name)
      File "/Users/zsailer/Documents/Work/oss/internal/jupyter_server/jupyter_server/extension/utils.py", line 55, in get_metadata
        module = importlib.import_module(package_name)
      File "/Users/env/lib/python3.8/importlib/__init__.py", line 127, in import_module
        return _bootstrap._gcd_import(name[level:], package, level)
      File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
      File "<frozen importlib._bootstrap>", line 991, in _find_and_load
      File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
      File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
      File "<frozen importlib._bootstrap_external>", line 843, in exec_module
      File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
      File "/Users/env/lib/python3.8/site-packages/jupyter_server_terminals/__init__.py", line 9, in <module>
        raise RuntimeError("Jupyter Server Terminals requires Jupyter Server 2.0+")
    RuntimeError: Jupyter Server Terminals requires Jupyter Server 2.0+
    jupyterlab enabled
    - Validating jupyterlab...
      jupyterlab 3.5.0 OK
    nbclassic disabled
    - Validating nbclassic...
      nbclassic  OK

[codecov]

Codecov Report

Base: 79.87% // Head: 79.93% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (b3d9e36) compared to base (2dd116f).
Patch coverage: 63.63% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1139      +/-   ##
==========================================
+ Coverage   79.87%   79.93%   +0.05%     
==========================================
  Files          68       68              
  Lines        8060     8067       +7     
  Branches     1598     1599       +1     
==========================================
+ Hits         6438     6448      +10     
+ Misses       1198     1197       -1     
+ Partials      424      422       -2     

Impacted Files	Coverage Δ
jupyter_server/extension/serverextension.py	85.71% <63.63%> (-1.43%)	⬇️
jupyter_server/serverapp.py	79.05% <0.00%> (+0.08%)	⬆️
...ter_server/services/kernels/connection/channels.py	59.60% <0.00%> (+1.10%)	⬆️

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[blink1073]

I'd say we put the stack trace behind a log.debug, we shouldn't show stack traces to the user by default.

[Zsailer]

Done 👍

[blink1073]

Thanks!

[blink1073]

@meeseeksdev please backport to 1.x