Missing track parameter in sources should raise appropriate error

Background

Tried building something like the following:

kind: autotools

sources:
- kind: git
  url: https://github.com/foo/bar.git
  ref:

It's missing the track parameter, so it outputs:

Inconsistent pipeline

    Exact versions are missing for the following elements
    Try tracking these elements first with `bst track`

      baz.bst

It should instead tell the user that the track parameter is missing as the suggested command does nothing to resolve the situation.

Task description

• Error message telling the user to include a track
• Unittested