Can be called on variables and expressions of the error type.

• Replaces the err variable with the following construction:

  if err != nil {
      return value1, value2, ...
    }

• Replaces the expr expression with the following construction:

  if err := expr; err != nil {
      return value1, value2, ...
    }

Each value is either the err variable or a default value of the corresponding return type of the enclosing function. You can call this template on expressions with multiple values.