fix: report a clear error when config has no [requirements] section

[grische]

mirror() crashed with a bare KeyError when the (hand-written) config file was missing the [requirements] section. Raise a ValueError with a message pointing at the config file instead.

[philnewm]

Agreed to have a preflight check for this

[philnewm]

@grische please rebase to ensure the merge conflict gets resolved

[github-actions]

Review

Small, well-targeted fix. mirror() previously crashed with a bare KeyError when morgan.ini had no [requirements] section; this raises a ValueError naming the offending config file instead.

Code quality

• The fix follows the existing codebase convention exactly (msg = f"..."; raise ValueError(msg), e.g. morgan/__init__.py:570-581), and correctly places the check in the module-level mirror() function rather than in Mirrorer.__init__, since copy_server also constructs a Mirrorer without needing a [requirements] section. Good scoping.

Error handling / UX note (non-blocking)

• main() doesn't wrap mirror(args) in a try/except (morgan/__init__.py:1001-1002), so the ValueError still propagates as an uncaught exception with a full traceback to the CLI user — same as the adjacent argparse.ArgumentTypeError a few lines above for missing config files. That's consistent with existing behavior, so not a regression, but if the goal is a clean CLI error message (not just a clearer exception type), consider catching config-related ValueErrors in main() and printing them without a traceback. Worth a follow-up if that's the actual pain point, but out of scope for this PR as titled.

Tests

• New TestMirrorConfigValidation test is well-isolated, reuses the project's established tmp_path/config-writing idiom (matches TestMirrorDependencyErrors at tests/test_init.py:590+), and asserts on the specific error message via pytest.raises(..., match=...). Good coverage for the fix.
• Minor nit: the test imports the whole morgan module just to call morgan.mirror(args), while the rest of the file imports names directly from morgan (from morgan import (..., Mirrorer, ...)). Could import mirror directly for consistency, though this is trivial.

Documentation

• No README/CHANGELOG changes — appropriate for a bug-fix-only PR of this size; nothing user-facing to document beyond the improved error message itself.

Security / performance

• No concerns — this only adds a config validation check with no new I/O, external input handling, or hot-path changes.

Overall: correct, minimal, consistent with project conventions. Approve.

[philnewm]

@grische feel free to decide if this should be included here or not

main() doesn't wrap mirror(args) in a try/except (morgan/init.py:1001-1002), so the ValueError still propagates as an uncaught exception with a full traceback to the CLI user — same as the adjacent argparse.ArgumentTypeError a few lines above for missing config files. That's consistent with existing behavior, so not a regression, but if the goal is a clean CLI error message (not just a clearer exception type), consider catching config-related ValueErrors in main() and printing them without a traceback. Worth a follow-up if that's the actual pain point, but out of scope for this PR as titled.