fix(Query): Do an error check before bubbling up nil error

[all-seeing-code]

Problem

Without the check, the code erraneously bubbles up nil error even when there's been an error in one of the child subgraph. This later causes a panic because the downstream code expects no error.

Reproduce this:

DQL Query:

{
    var(func: eq(name, "Alicey")){
        friend(orderdesc: id) {
            f as count(uid)
        }
    }
    q(func: uid(f)){
        name
    } 
}

Fixes: https://github.com/dgraph-io/projects-internal/issues/89

Solution

Add an error check.

[skrdgraph]

Looks good 👍

[harshil-goel]

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