Raise and reraise exceptions with Stdlib rather than Lwt

[MisterDA]

Lwt's documentation reads:

In most cases, it is better to use failwith s from the standard
library.

and

Whenever possible, it is recommended to use raise exn instead, as
raise captures a backtrace, while Lwt.fail does not. If you call
raise exn in a callback that is expected by Lwt to return a
promise, Lwt will automatically wrap exn in a rejected promise,
but the backtrace will have been recorded by the OCaml runtime.

For example, bind's second argument is a callback which returns a
promise. And so it is recommended to use raise in the body of that
callback.

Use Lwt.fail only when you specifically want to create a rejected
promise, to pass to another function, or store in a data structure.

Prefer to capture backtraces to improve debugability.

See also ocsigen/lwt#1008 and mirage/ocaml-conduit#430.