config: `dvc config` doesn't throw an error when used outside of a dvc repository without --system or --global

```
DVC version: 0.52.1+1e9f1d
Python version: 3.7.3
Platform: Linux-5.2.1-arch1-1-ARCH-x86_64-with-arch
Binary: False
Filesystem type (workspace): ('xfs', '/dev/mapper/vg-root')
```
---

```bash
rm -rf .dvc
dvc remote modify something whatever yeah
```

#### Expected
```
ERROR: you are not inside of a dvc repository (checked up to mount point '/')
```