ExtensionPoint: return True on successful validate()

[minrk]

Related to #500, but doesn't actually fix the main issue there (failure to identify extensions that are not importable).

Current situation:

• ExtensionPoint.validate always returns a falsy value (None or False)
• ExtensionPackage only checks the falsiness, which means it always returns False
• ListServerExtensionApp never checked validate return value, meaning all extensions were always considered valid, even though all extensions actually self-reported as invalid

This fixes:

• ExtensionPoint.validate returns True on successful validation
• ListServerExtensionApp checks the return value of validate

The original error causing failed validation is still swallowed and not logged, which should probably be addressed at some point as well, but at least now values are correct.

This came up when I was trying to write some tests validating an image, where I had:

def test_extensions():
    from jupyter_core.paths import jupyter_config_path
    from jupyter_server.extension.manager import ExtensionManager
    from jupyter_server.extension.config import ExtensionConfigManager

    m = ExtensionManager(
        config_manager=ExtensionConfigManager(read_config_paths=jupyter_config_path())
    )
    for name, ext in m.extensions.items():
        print(f"Validating extension {name}")
        assert ext.validate()

and was surprised to see that all the extensions were actually invalid.

[codecov-commenter]

Codecov Report

Merging #503 (0a86f7c) into master (5e77b73) will increase coverage by 0.05%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
+ Coverage   77.68%   77.73%   +0.05%     
==========================================
  Files         106      106              
  Lines        9276     9280       +4     
  Branches     1001     1002       +1     
==========================================
+ Hits         7206     7214       +8     
+ Misses       1711     1706       -5     
- Partials      359      360       +1     

Impacted Files	Coverage Δ
jupyter_server/extension/serverextension.py	75.35% <0.00%> (-0.54%)	⬇️
jupyter_server/extension/manager.py	89.56% <100.00%> (+2.82%)	⬆️
jupyter_server/tests/extension/test_manager.py	100.00% <100.00%> (ø)

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[Zsailer]

Thanks, @minrk. LGTM.