[#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/local_object_storage/engine/put.go

@@ -143,7 +143,7 @@ func (e *StorageEngine) putToShard(ctx context.Context, sh hashedShard, pool uti
 			} else {
 				e.log.Warn(ctx, logs.EngineCouldNotCheckObjectExistence,
 					zap.Stringer("shard_id", sh.ID()),
-					zap.String("error", err.Error()),
+					zap.Error(err),
 					zap.String("trace_id", tracingPkg.GetTraceID(ctx)))
 			}
 
@@ -165,14 +165,14 @@ func (e *StorageEngine) putToShard(ctx context.Context, sh hashedShard, pool uti
 				errors.Is(err, common.ErrReadOnly) || errors.Is(err, common.ErrNoSpace) {
 				e.log.Warn(ctx, logs.EngineCouldNotPutObjectToShard,
 					zap.Stringer("shard_id", sh.ID()),
-					zap.String("error", err.Error()),
+					zap.Error(err),
 					zap.String("trace_id", tracingPkg.GetTraceID(ctx)))
 				return
 			}
 			if client.IsErrObjectAlreadyRemoved(err) {
 				e.log.Warn(ctx, logs.EngineCouldNotPutObjectToShard,
 					zap.Stringer("shard_id", sh.ID()),
-					zap.String("error", err.Error()),
+					zap.Error(err),
 					zap.String("trace_id", tracingPkg.GetTraceID(ctx)))
 				res.status = putToShardRemoved
 				res.err = err