Give a better error message when users use a ? or + on subvariables.

[Paul-Ferrell]

Can I not do something like:

_base:
  variables:
    hpcc:
      compiler?: gcc/10.3.1
      mpi?: openmpi/4.1.2

The above isn't allowed. This is expected:

_base:
  variables:
    hpcc?:
      compiler: gcc/10.3.1
      mpi: openmpi/4.1.2

[Paul-Ferrell]

As noted by @nicholas-sly