Add ShadeError base exception class

[Errordog2]

Summary

• add shade.errors with a common ShadeError base class carrying message, optional HTTP status, and raw response body
• add SDK-specific subclasses for authentication, invalid request, not found, and network failures
• export the error classes from the package root so callers can catch shade.ShadeError

Validation

• python -m pytest tests/test_errors.py tests/test_gateway.py -q (6 passed)
• python -m compileall -q src tests
• git diff --check

Closes #15

[codebestia]

LGTM!
Thank you for your contribution @Errordog2