[#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/blobstor/blobovniczatree/iterate.go

@@ -44,7 +44,7 @@ func (b *Blobovniczas) Iterate(ctx context.Context, prm common.IteratePrm) (comm
 				if prm.IgnoreErrors {
 					b.log.Warn(ctx, logs.BlobstorErrorOccurredDuringTheIteration,
 						zap.Stringer("address", elem.Address()),
-						zap.String("err", err.Error()),
+						zap.Error(err),
 						zap.String("storage_id", p),
 						zap.String("root_path", b.rootPath))
 					return nil
@@ -77,7 +77,7 @@ func (b *Blobovniczas) iterateBlobovniczas(ctx context.Context, ignoreErrors boo
 		if err != nil {
 			if ignoreErrors {
 				b.log.Warn(ctx, logs.BlobstorErrorOccurredDuringTheIteration,
-					zap.String("err", err.Error()),
+					zap.Error(err),
 					zap.String("storage_id", p),
 					zap.String("root_path", b.rootPath))
 				return false, nil