[#1502] Use zap.Error for logging errors

Use `zap.Error` instead of `zap.String` for logging errors: change all expressions like
`zap.String("error", err.Error())` or `zap.String("err", err.Error())` to `zap.Error(err)`.
Leave similar expressions with other messages unchanged, for example,
`zap.String("last_error", lastErr.Error())` or `zap.String("reason", ctx.Err().Error())`.

This change was made by applying the following patch:
```diff
@@
var err expression
@@
-zap.String("error", err.Error())
+zap.Error(err)

@@
var err expression
@@
-zap.String("err", err.Error())
+zap.Error(err)
```

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/innerring/processors/netmap/process_epoch.go

@@ -17,7 +17,7 @@ func (np *Processor) processNewEpoch(ctx context.Context, ev netmapEvent.NewEpoc
 	epochDuration, err := np.netmapClient.EpochDuration()
 	if err != nil {
 		np.log.Warn(ctx, logs.NetmapCantGetEpochDuration,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 	} else {
 		np.epochState.SetEpochDuration(epochDuration)
 	}
@@ -28,19 +28,19 @@ func (np *Processor) processNewEpoch(ctx context.Context, ev netmapEvent.NewEpoc
 	if err != nil {
 		np.log.Warn(ctx, logs.NetmapCantGetTransactionHeight,
 			zap.String("hash", ev.TxHash().StringLE()),
-			zap.String("error", err.Error()))
+			zap.Error(err))
 	}
 
 	if err := np.epochTimer.ResetEpochTimer(h); err != nil {
 		np.log.Warn(ctx, logs.NetmapCantResetEpochTimer,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 	}
 
 	// get new netmap snapshot
 	networkMap, err := np.netmapClient.NetMap()
 	if err != nil {
 		np.log.Warn(ctx, logs.NetmapCantGetNetmapSnapshotToPerformCleanup,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 
 		return false
 	}