net/http: ListenAndServe examples are inconsistent about checking err != nil

[adonovan]

The doc comment at http.ListenAndServe says:

    err := http.ListenAndServe(":12345", nil)
    if err != nil {
        log.Fatal("ListenAndServe: ", err)
    }

whereas other examples in the same package say:

    log.Fatal(http.ListenAndServe(":12345", nil))

These shorter forms, though pleasingly succinct, rely on the undocumented assumption that if ListenAndServe ever returns, it returns non-nil.

The examples should either always check for nil, or ListenAndServe should document the non-nil postcondition.

[bradfitz]

We could also document that ListenAndServe ways returns non-nil if we don't already.

[gopherbot]

CL https://golang.org/cl/13780 mentions this issue.