docs(rockandroll): upgrade instructions

subjects/rockandroll/README.md

@@ -7,6 +7,8 @@ Write a function called `RockAndRoll()` that takes an `int` and returns a `strin
 - If the number is divisible by 2, print `rock` followed by a newline `\n`.
 - If the number is divisible by 3, print `roll` followed by a newline `\n`.
 - If the number is divisible by 2 and 3, print `rock and roll` followed by a newline `\n`.
+- If the number is negative return `error: number is negative` followed by a newline `\n`.
+- If the number is non divisible return `error: non divisible` followed by a newline `\n`.
 
 ### Expected function
 
@@ -23,14 +25,14 @@ Here is a possible program to test your function:
 package main
 
 import (
-        "fmt"
-        "piscine"
+	"fmt"
+	"piscine"
 )
 
 func main() {
-        fmt.Println(piscine.RockAndRoll(4))
-        fmt.Println(piscine.RockAndRoll(9))
-        fmt.Println(piscine.RockAndRoll(6))
+	fmt.Println(piscine.RockAndRoll(4))
+	fmt.Println(piscine.RockAndRoll(9))
+	fmt.Println(piscine.RockAndRoll(6))
 }
 ```
 And its output: