perf/app: log errors in /cron/syncinflux

Change-Id: Ie6be018b761afcb5684621eeb08cf52aab7d1846
Reviewed-on: https://go-review.googlesource.com/c/build/+/624436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

perf/app/influx.go

@@ -67,6 +67,7 @@ func (a *App) syncInflux(w http.ResponseWriter, r *http.Request) {
 
 	lastPush, err := latestInfluxTimestamp(ctx, ifxc)
 	if err != nil {
+		log.Printf("Error getting latest Influx timestamp: %v", err)
 		http.Error(w, err.Error(), 500)
 		return
 	}
@@ -79,6 +80,7 @@ func (a *App) syncInflux(w http.ResponseWriter, r *http.Request) {
 
 	uploads, err := a.uploadsSince(ctx, lastPush)
 	if err != nil {
+		log.Printf("Error getting new uploads: %v", err)
 		http.Error(w, err.Error(), 500)
 		return
 	}