! exec karma --config.file=karma.yaml
! stdout .
cmp stderr stderr.txt

-- stderr.txt --
level=ERROR msg="Execution failed" error="authentication.header.group_value_re is required when authentication.header.group_name is set"
-- karma.yaml --
alertmanager:
  servers:
    - name: default
      uri: https://127.0.0.1:9093
authentication:
  header:
    group_name: X-Groups
