configs: A better error message for custom variable validation
configs: A better error message for custom variable validation

Our previous message conflated the requirement for a full sentence with
the suggestion to write in a style similar to Terraform's built-in error
messages, which created a sense that the system would actively reject an
error message written in another language.

There's no intent here to block writing error messages in other languages,
but there is a practical consideration that Terraform's UI output is
currently not localized and so consistency with Terraform's other output,
if that's important to a module author, will typically mean writing the
error message in English.

ewbankkit/terraform,apparentlymart/terraform,rnaveiras/terraform,ewbankkit/terraform,hashicorp/terraform,hashicorp/terraform,tommynsong/terraform,tommynsong/terraform,rnaveiras/terraform,ewbankkit/terraform,rnaveiras/terraform,apparentlymart/terraform,apparentlymart/terraform,hashicorp/terraform,apparentlymart/terraform,apparentlymart/terraform,tommynsong/terraform,ewbankkit/terraform