cmd/sunlight: print a helpful error if the inception date is wrong

Fixes #26

cmd/sunlight/main.go

@@ -26,6 +26,7 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"encoding/pem"
+	"errors"
 	"flag"
 	"io"
 	"log/slog"
@@ -434,7 +435,10 @@ func main() {
 		}
 
 		l, err := ctlog.LoadLog(ctx, cc)
-		if err != nil {
+		if errors.Is(err, ctlog.ErrLogNotFound) {
+			fatalError(logger, "log not found, but today is not the Inception date",
+				"today", time.Now().Format(time.DateOnly), "inception", lc.Inception)
+		} else if err != nil {
 			fatalError(logger, "failed to load log", "err", err)
 		}
 		defer l.CloseCache()

internal/ctlog/ctlog.go

@@ -392,6 +392,8 @@ var optsStaging = &UploadOptions{Compress: true}
 var optsIssuer = &UploadOptions{ContentType: "application/pkix-cert", Immutable: true}
 var optsCheckpoint = &UploadOptions{ContentType: "text/plain; charset=utf-8"}
 
+var ErrLogNotFound = errors.New("log not found")
+
 // A LockBackend is a database that supports compare-and-swap operations.
 //
 // It is shared across multiple Log instances, and is used only to store the
@@ -401,6 +403,8 @@ var optsCheckpoint = &UploadOptions{ContentType: "text/plain; charset=utf-8"}
 type LockBackend interface {
 	// Fetch obtains the current checkpoint for a given log, as well as the data
 	// necessary to perform a compare-and-swap operation.
+	//
+	// It must return ErrLogNotFound if the log doesn't exist.
 	Fetch(ctx context.Context, logID [sha256.Size]byte) (LockedCheckpoint, error)
 
 	// Replace uploads a new checkpoint, atomically checking that the old

internal/ctlog/dynamodb.go

@@ -3,7 +3,6 @@ package ctlog
 import (
 	"context"
 	"crypto/sha256"
-	"errors"
 	"fmt"
 	"log/slog"
 	"net/http"
@@ -91,7 +90,7 @@ func (b *DynamoDBBackend) Fetch(ctx context.Context, logID [sha256.Size]byte) (L
 		return nil, err
 	}
 	if resp.Item == nil {
-		return nil, errors.New("checkpoint not found")
+		return nil, ErrLogNotFound
 	}
 	return &dynamoDBCheckpoint{logID: logID,
 		body: resp.Item["checkpoint"].(*types.AttributeValueMemberB).Value}, nil