feat: Explicit error with unsupported files

[krlmlr]

For #5.

When I try to run tests, I see:

✗ make test_debug
Running DEBUG tests..
/Users/kirill/git/duckdb-read-stat/configure/venv/bin/python3: No module named duckdb_sqllogictest.__main__; 'duckdb_sqllogictest' is a package and cannot be directly executed
make: *** [test_extension_debug_internal] Error 1

Happy to add tests once I understand how to run them, perhaps also on CI/CD?

[krlmlr]

Thanks, fixed. Would you like to share advice on running the tests locally?