metrics diff: cryptic error in --no-scm projects [qa]

```console
λ dvc version
DVC version: 0.92.0
Python version: 3.7.5
Platform: Windows-10-10.0.18362-SP0
Binary: True
Package: exe

λ dvc metrics diff
ERROR: unexpected error - 'NoSCM' object has no attribute 'resolve_rev'
```

Run in a --no-scm DVC project. Compare to `dvc diff`:

```console
λ dvc diff
ERROR: failed to get diff - only supported for Git repositories
```