Investigate exception error when saving Job without associated repo

Steps to reproduce:

1) Try to create a job without a repository associated.

elixir
```
ElixirBench.Benchmarks.create_job(%ElixirBench.Repos.Repo{}, %{branch_name: "mm/benche", commit_sha: "ABC123"})
```

2) This raises a Postgrex.Error exception, but should return ```{:error, reason}``` requiring a valid ```repo_id```  to create the Job