refactor(dataset): Tick takes an onError callback, no more stderr

Libraries shouldn't decide where errors go. Tick now passes Load
errors to onError (nil to ignore); callers pick log/count/page.
check-ip supplies its own stderr writer.

cmd/check-ip/main.go

@@ -246,7 +246,9 @@ func newChecker(ctx context.Context, cfg Config) (*Checker, func(), error) {
 	}
 	fmt.Fprintf(os.Stderr, "Loaded inbound=%d outbound=%d\n",
 		inbound.Value().Size(), outbound.Value().Size())
-	go group.Tick(ctx, refreshInterval)
+	go group.Tick(ctx, refreshInterval, func(err error) {
+		fmt.Fprintf(os.Stderr, "refresh: %v\n", err)
+	})
 
 	whitelist, err := loadWhitelist(cfg.Whitelist)
 	if err != nil {

sync/dataset/dataset.go

@@ -16,8 +16,6 @@ package dataset
 
 import (
 	"context"
-	"fmt"
-	"os"
 	"sync/atomic"
 	"time"
 )
@@ -81,9 +79,10 @@ func (g *Group) Load(ctx context.Context) error {
 	return nil
 }
 
-// Tick calls Load every interval until ctx is done. Errors are written to
+// Tick calls Load every interval until ctx is done. Load errors are passed to
-// stderr and do not stop the loop.
+// onError (if non-nil) and do not stop the loop; callers choose whether to log,
-func (g *Group) Tick(ctx context.Context, interval time.Duration) {
+// count, page, or ignore. Run in a goroutine: `go g.Tick(ctx, d, onError)`.
+func (g *Group) Tick(ctx context.Context, interval time.Duration, onError func(error)) {
 	t := time.NewTicker(interval)
 	defer t.Stop()
 	for {
@@ -91,8 +90,8 @@ func (g *Group) Tick(ctx context.Context, interval time.Duration) {
 		case <-ctx.Done():
 			return
 		case <-t.C:
-			if err := g.Load(ctx); err != nil {
+			if err := g.Load(ctx); err != nil && onError != nil {
-				fmt.Fprintf(os.Stderr, "dataset: load error: %v\n", err)
+				onError(err)
 			}
 		}
 	}