[#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/exists.go

@@ -57,7 +57,7 @@ func (b *Blobovniczas) Exists(ctx context.Context, prm common.ExistsPrm) (common
 			if !client.IsErrObjectNotFound(err) {
 				b.log.Debug(ctx, logs.BlobovniczatreeCouldNotGetObjectFromLevel,
 					zap.String("level", p),
-					zap.String("error", err.Error()),
+					zap.Error(err),
 					zap.String("trace_id", tracingPkg.GetTraceID(ctx)))
 			}
 		}