Fix _scheduler/docs/... path 500 errors

[nickva]

Return a 400 response for _scheduler/docs/foo or _scheduler/docs/w/x/_replicator/y/z paths instead of a 500 error.

Fix: #5089

[rnewson]

curl https://foo:bar@localhost:15984/_scheduler/docs/rep2 -i
HTTP/1.1 400 Bad Request
Cache-Control: must-revalidate
Content-Length: 36
Content-Type: application/json
Date: Tue, 25 Jun 2024 09:25:15 GMT
Server: CouchDB/3.3.3-03c4a6a (Erlang OTP/25)
X-Couch-Request-ID: 18b134a7b5
X-CouchDB-Body-Time: 0

{"error":"bad_request","reason":""}

This is definitely an improvement over

{"error":"unknown_error","reason":"function_clause","ref":1083485994}

but I think we should fill in a reason.

[nickva]

Good point, I'll add a reason to the bad_request.

[nickva]

% http $DB/_scheduler/docs/w/x/_replicator/y/z
HTTP/1.1 400 Bad Request

{
    "error": "bad_request",
    "reason": "Invalid path"
}

% http $DB/_scheduler/docs/foo
HTTP/1.1 400 Bad Request

{
    "error": "bad_request",
    "reason": "Invalid path"
}