Add RegistryError base with message, details, and original_reference fields.
Implement all 9 typed exception types per Package Registry Standard §13.2:
PackageNotFoundError, InvalidPackageIdError, InvalidPackageReferenceError,
VersionNotFoundError, ValidationError, AuthenticationRequiredError,
AccessDeniedError, ConflictError, and RegistryNetworkError.
RegistryNetworkError carries status_code and url for transport failures.
__str__ includes original_reference when available.
exception_for_status() maps HTTP codes to typed exceptions.
_ERROR_TYPE_MAP enables error-type parsing from JSON error bodies.
23 Behave BDD scenarios in features/registry_exceptions.feature.
12 Robot Framework integration tests in robot/exceptions.robot.
Existing registry_http_client tests updated for InternalServerError removal.
ISSUES CLOSED: #29
CoreRasurae
