Specific error message when extra-deps git commit is empty.

Could we do better with this error message?

```
$ stack build --dry-run
...
Error: [S-6602]
Stack could not load and parse .../stack.yaml as
a YAML configuraton file.

While loading and parsing, Stack encountered the following error:

Aeson exception:
Error in $['extra-deps'][23]: failed to parse field 'extra-deps':
parsing Text failed, expected String, but encountered Object

For help about the content of Stack's YAML configuration files,
see (for the most recent release of Stack)
http://docs.haskellstack.org/en/stable/yaml_configuration/.
```

With the following `stack.yaml` snippet we could be saying that the commit hash is empty:

```
extra-deps:
- git: https://github.com/space-vacuum/squeal
commit:
subdirs:
- squeal-postgresql
- squeal-postgresql-ltree
```

The `23` of `Error in $['extra-deps'][23]` is not one of the line numbers of this git dependency.