class DataLayerError(Exception):
    """Base exception for the data layer."""


class DependencyError(DataLayerError):
    """Raised when an optional runtime dependency is required but missing."""


class ProviderError(DataLayerError):
    """Raised when a provider request fails or returns unusable data."""


class UnknownInstrumentError(DataLayerError):
    """Raised when the requested instrument is not configured."""


class MissingLayerError(DataLayerError):
    """Raised when a repair operation cannot find its source layer."""