lib/plugins/aws/rollbackFunction/index.js/AwsRollbackFunction/getFunctionToBeRestored
class AwsRollbackFunction:  getFunctionToBeRestored() {
    const funcName = this.options.function;
    let funcVersion = this.options.version;

    // versions need to be string so that AWS understands it
    funcVersion = String(this.options.version);

    this.serverless.cli.log(`Rolling back function "${funcName}" to version "${funcVersion}"...`);

    const funcObj = this.serverless.service.getFunction(funcName);

    const params = {
      FunctionName: funcObj.name,
      Qualifier: funcVersion,
    };

    return this.provider
      .request('Lambda', 'getFunction', params)
      .then(func => func)
      .catch(error => {
        if (error.message.match(/not found/)) {
          const errorMessage = [
            `Function "${funcName}" with version "${funcVersion}" not found.`,
            ` Please check if you've deployed "${funcName}"`,
            ` and version "${funcVersion}" is available for this function.`,
            ' Please check the docs for more info.',
          ].join('');
          throw new Error(errorMessage);
        }
        throw new Error(error.message);
      });
  }