http: improve 404 Not Found response message

[jkirschner-hashicorp]

When a URL path is not found, return a non-empty message with the 404 status code to help the user understand what went wrong. If the URL path was not prefixed with '/v1/', suggest that may be the cause of the problem (which is a common mistake).

Examples

Path not prefixed with '/v1/`

curl --request GET --verbose localhost:8500/agent/services
Note: Unnecessary use of -X or --request, GET is already inferred.
*   Trying 127.0.0.1:8500...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8500 (#0)
> GET /agent/services HTTP/1.1
> Host: localhost:8500
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 404 Not Found
< Date: Mon, 13 Dec 2021 16:32:45 GMT
< Content-Length: 87
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host localhost left intact
Invalid URL path: if attempting to use the HTTP API, ensure the path starts with '/v1/'

Path prefixed with '/v1/`

# endpoint is misspelled
$ curl --request GET localhost:8500/v1/agent/srvices
Invalid URL path: not a recognized HTTP API endpoint

[dnephin]

Looks good! I think this should help make it easier to notice the problem.

When we start introducing different versions I think we'll want to update our API docs to include the /v1 prefix in the docs, which should also help.

[dnephin]

minor: maybe a trailing newline on these?

Suggested change

	fmt.Fprint(resp, "Invalid URL path: not a recognized HTTP API endpoint")
	} else {
	fmt.Fprint(resp, "Invalid URL path: if attempting to use the HTTP API, ensure the path starts with '/v1/'")
	fmt.Fprintln(resp, "Invalid URL path: not a recognized HTTP API endpoint")
	} else {
	fmt.Fprintln(resp, "Invalid URL path: if attempting to use the HTTP API, ensure the path starts with '/v1/'")

[jkirschner-hashicorp]

Good catch, I'll make this change then merge

[jkirschner-hashicorp]

we'll want to update our API docs to include the /v1 prefix in the docs, which should also help.

@dnephin: I made this mistake relatively often when starting out for that reason... the docs omit the /v1 prefix, so unless you consistently remember it's needed, you'll run into this and be confused. (And I still see it happen with people who aren't new.)

[hc-github-team-consul-core]

🍒 If backport labels were added before merging, cherry-picking will start automatically.

To retroactively trigger a backport after merging, add backport labels and re-run https://circleci.com/gh/hashicorp/consul/527069.

[hc-github-team-consul-core]

🍒✅ Cherry pick of commit f81dd81 onto release/1.11.x succeeded!