A protocol-agnostic type representing an internal framework error. As of writing, this can only
occur upon failure to extract an crate::extension::Extension from the request.
This type is converted into protocol-specific error variants. For example, in the
[crate::protocol::rest_json_1] protocol, it is converted to the
[crate::protocol::rest_json_1::runtime_error::RuntimeError::InternalFailure] variant.