lib/classes/Variables.js/Variables/getValueFromSource
class Variables:  getValueFromSource(variableString) {
    if (variableString.match(this.envRefSyntax)) {
      return this.getValueFromEnv(variableString);
    } else if (variableString.match(this.optRefSyntax)) {
      return this.getValueFromOptions(variableString);
    } else if (variableString.match(this.selfRefSyntax)) {
      return this.getValueFromSelf(variableString);
    } else if (variableString.match(this.fileRefSyntax)) {
      return this.getValueFromFile(variableString);
    } else if (variableString.match(this.cfRefSyntax)) {
      return this.getValueFromCf(variableString);
    } else if (variableString.match(this.s3RefSynax)) {
      return this.getValueFromS3(variableString);
    }
    const errorMessage = [
      `Invalid variable reference syntax for variable ${variableString}.`,
      ' You can only reference env vars, options, & files.',
      ' You can check our docs for more info.',
    ].join('');
    throw new this.serverless.classes.Error(errorMessage);
  }