Previously, if decoding the job, tasks, or vault portion of the config
failed, we would not return an error; it was silently ignored.
This also includes a little refactor to reduce some duplication.
The HCL2 parser needs to apply special parsing tweaks so it can parse
the task config the same way as HCL1. Particularly, it needs to
reinterprets `map[string]interface{}` fields and blocks that appear when
attributes are expected.
This commit restricts the special casing to the Job fields, and ignore
`variables` and `locals` block.
