Better error message for missing `rescript.json`

In a project using a dependency that still has no `rescript.json` (but only `bsconfig.json`), I get the following error:

```
ERROR:
We could not build package tree  '@glennsl/rescript-fetch', at path '/Users/christoph/projects/xxx'. Error: No such file or directory (os error 2)
```

The error should state explicitly that rescript.json was not found.