Better error message when parsing invalid variant_config

[nichmor]

Having this variant config:

python:
  - "3.11.* *cpython"
  # - "3.12.* *cpython"
  # - "3.9.* *cpython"


cxx_compiler:
  - if win:
    then: vs2019

I will receive following error message:

rattler-build build -r mamba -m mamba/variant_config.yaml

 ╭─ Finding outputs from recipe
 │
 ╰─────────────────── (took 0 seconds)
Error:   × Failed to parse recipe

I think it's better to point where variant_config is wrong