`DebuggerInitializationError` kills the kernel

[krassowski]

When debugger fails to intialise, the kernel dies because DebuggerInitializationError does not get handled on import. See jupyterlab/jupyterlab#13069 for an example on how innocent code.py file might prevent user from launching the kernel.

Example traceback:

Traceback (most recent call last):
  File "/3.9.5/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/3.9.5/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/ipykernel_launcher.py", line 15, in <module>
    from ipykernel import kernelapp as app
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/ipykernel/kernelapp.py", line 51, in <module>
    from .ipkernel import IPythonKernel
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/ipykernel/ipkernel.py", line 19, in <module>
    from .debugger import Debugger, _is_debugpy_available
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/ipykernel/debugger.py", line 22, in <module>
    from debugpy.server import api  # noqa
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/debugpy/server/__init__.py", line 7, in <module>
    import debugpy._vendored.force_pydevd  # noqa
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/debugpy/_vendored/force_pydevd.py", line 28, in <module>
    pydevd_constants = import_module('_pydevd_bundle.pydevd_constants')
  File "/3.9.5/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_constants.py", line 379, in <module>
    from _pydev_bundle._pydev_saved_modules import thread, threading
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/debugpy/_vendored/pydevd/_pydev_bundle/_pydev_saved_modules.py", line 91, in <module>
    import code as _code;    verify_shadowed.check(_code, ['compile_command', 'InteractiveInterpreter'])
  File "/3.9.5/envs/ipython/lib/python3.9/site-packages/debugpy/_vendored/pydevd/_pydev_bundle/_pydev_saved_modules.py", line 75, in check
    raise DebuggerInitializationError(msg)
_pydev_bundle._pydev_saved_modules.DebuggerInitializationError: It was not possible to initialize the debugger due to a module name conflict.