Elastic scaling: use an assumed CoreIndex in candidate-backing

polkadot/node/core/backing/src/error.rs

@@ -48,6 +48,9 @@ pub enum Error {
 	#[error("Candidate is not found")]
 	CandidateNotFound,
 
+	#[error("CoreIndex cannot be determined for a candidate")]
+	CoreIndexUnavailable,
+
 	#[error("Signature is invalid")]
 	InvalidSignature,
 
@@ -60,6 +63,12 @@ pub enum Error {
 	#[error("Fetching validation code by hash failed {0:?}, {1:?}")]
 	FetchValidationCode(ValidationCodeHash, RuntimeApiError),
 
+	#[error("Fetching validator groups failed")]
+	FetchValidatorGroups(RuntimeApiError),
+
+	#[error("Fetching availability cores failed")]
+	FetchAvailabilityCores(RuntimeApiError),
+
 	#[error("Fetching Runtime API version failed {0:?}")]
 	FetchRuntimeApiVersion(RuntimeApiError),