report more details on build backend exception

[dimbleby]

show the inner error associated with a BuildBackendException

eg for #8137 (comment) this gives the extra output:

  Traceback (most recent call last):
    File "/home/dch/.virtualenvs/poetry/lib/python3.10/site-packages/pyproject_hooks/_in_process/_in_process.py", line 77, in _build_backend
      obj = import_module(mod_path)
            ^^^^^^^^^^^^^^^^^^^^^^^
    File "/usr/lib/python3.12/importlib/__init__.py", line 90, in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "<frozen importlib._bootstrap>", line 1381, in _gcd_import
    File "<frozen importlib._bootstrap>", line 1354, in _find_and_load
    File "<frozen importlib._bootstrap>", line 1304, in _find_and_load_unlocked
    File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
    File "<frozen importlib._bootstrap>", line 1381, in _gcd_import
    File "<frozen importlib._bootstrap>", line 1354, in _find_and_load
    File "<frozen importlib._bootstrap>", line 1325, in _find_and_load_unlocked
    File "<frozen importlib._bootstrap>", line 929, in _load_unlocked
    File "<frozen importlib._bootstrap_external>", line 994, in exec_module
    File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
    File "/tmp/tmp6wqbgy0u/.venv/lib/python3.12/site-packages/setuptools/__init__.py", line 10, in <module>
      import distutils.core
  ModuleNotFoundError: No module named 'distutils'

which makes it considerably more obvious what the problem is (distutils doesn't exist in python 3.12).

Maybe it'll show something helpful in #7611 too.

[github-actions]

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