lib/classes/Variables.js/Variables/warnIfNotFound
class Variables:  warnIfNotFound(variableString, valueToPopulate) {
    if (
      valueToPopulate === null ||
      typeof valueToPopulate === 'undefined' ||
      (typeof valueToPopulate === 'object' &&
        !Array.isArray(valueToPopulate) &&
        _.isEmpty(valueToPopulate))
    ) {
      let varType;
      if (variableString.match(this.envRefSyntax)) {
        varType = 'environment variable';
      } else if (variableString.match(this.optRefSyntax)) {
        varType = 'option';
      } else if (variableString.match(this.selfRefSyntax)) {
        varType = 'service attribute';
      } else if (variableString.match(this.fileRefSyntax)) {
        varType = 'file';
      } else if (variableString.match(this.ssmRefSyntax)) {
        varType = 'SSM parameter';
      }
      if (!_.isUndefined(varType)) {
        logWarning(
          `A valid ${varType} to satisfy the declaration '${variableString}' could not be found.`
        );
      }
    }
    return valueToPopulate;
  }