09_sumAll : Clarify negative input handling (#653)

Co-authored-by: mao-sz <122839503+mao-sz@users.noreply.github.com>

Author: shauryaa108

foundations/09_sumAll/README.md

@@ -6,7 +6,9 @@ Implement a function that takes 2 positive integers and returns the sum of every
 sumAll(1, 4) // returns the sum of 1 + 2 + 3 + 4 which is 10
 ```
 
+If the function receives invalid arguments (such as negative numbers, non-integers, strings etc. - anything other than positive integers), it should return the string `'ERROR'`.
+
 ## Hints
 
 - How will you ensure you're summing all integers within the correct range, no matter the order of the inputs?
-- Think about your sum's starting value. Then, how can you make sure every single number from the smaller input to the larger one (including both) gets added to it?
+- Think about your sum's starting value. Then, how can you make sure every single number from the smaller input to the larger one (including both) gets added to it?