Be strict about error paths format (#1073)

* Be strict about error paths

* Update spec/Section 7 -- Response.md

Co-authored-by: Benjie <benjie@jemjie.com>

* format

---------

Co-authored-by: Benjie <benjie@jemjie.com>

Author: martinbonnin

spec/Section 7 -- Response.md

@@ -109,11 +109,11 @@ must contain an entry with the key `path` that details the path of the response
 field which experienced the error. This allows clients to identify whether a
 `null` result is intentional or caused by a runtime error.
 
-This field should be a list of path segments starting at the root of the
-response and ending with the field associated with the error. Path segments that
-represent fields should be strings, and path segments that represent list
-indices should be 0-indexed integers. If the error happens in an aliased field,
-the path to the error should use the aliased name, since it represents a path in
+If present, this field must be a list of path segments starting at the root of
+the response and ending with the field associated with the error. Path segments
+that represent fields must be strings, and path segments that represent list
+indices must be 0-indexed integers. If the error happens in an aliased field,
+the path to the error must use the aliased name, since it represents a path in
 the response, not in the request.
 
 For example, if fetching one of the friends' names fails in the following