unless is inconsistent when there are unresolved variables

Because of some mistakes / bugs in the code, we ended up in a place where unless is inconsistent:

This is fine, the promise will be evaluated:

bundle agent main
{
  vars:
    "a"
      string => "value",
      unless => "$(no_such_var)";
}

This is not fine, the promise will cause a fatal error:

bundle agent main
{
  vars:
    "a"
      string => "value",
      unless => isvariable("$(no_such_var)");
}

Based on the discussion in this ticket:

https://tracker.mender.io/browse/CFE-2689

We agreed to keep this error message, for now, but it might be appropriate to remove the error so the 2 cases are consistent in this edge case.

This ticket discusses the bigger problem:

https://tracker.mender.io/browse/CFE-3160

Some users thought it was better to keep this policy not allowed (using fatal error) until that bigger long term change is done.