better error messages while loading configuration extends

[purajit]

Improves error messages during errors caused while extending a configuration using extend.

In the case of circular dependencies, it actually fixes the error too!

Old error messages:

# circular dependency - without even using pyproject.toml!
ruff failed
  Cause: Circular dependency detected in pyproject.toml

# file in "extends" doesn't exist
ruff failed
  Cause: Failed to read <path>/ruff4.toml
  Cause: No such file or directory (os error 2)

Now:

# circular dependency
ruff failed
  Cause: Circular dependency detected: <path>/ruff.toml -> <path>/ruff2.toml -> <path>/ruff3.toml -> <path>/ruff.toml

# circular dependency with a single file!
ruff failed
  Cause: Circular dependency detected: <path>/ruff.toml -> <path>/ruff.toml

# file in "extends" doesn't exist
ruff failed
  Cause: Failed to load last configuration in chain: <path>/ruff.toml -> <path>/ruff2.toml -> <path>/ruff3.toml -> <path>/ruff4.toml
  Cause: Failed to read <path>/ruff4.toml
  Cause: No such file or directory (os error 2)

[github-actions]

ruff-ecosystem results

Linter (stable)

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Linter (preview)

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Formatter (stable)

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Formatter (preview)

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[MichaReiser]

Nice, this is a great improvement. I've some suggestions on how we can improve the message further. It would probably also be good to add a CLI test for it, similar to what we have here