feat(users/Profpatsch): add die_* helpers for semantic exit errors

There is this semantic exit code schema championed by execline and
skaware tooling, and we refined and documented it a bit in lorri
d1d673d420/src/ops/mod.rs (L24-L35)
in the past.

This just transcribes the error messages into simple helper functions.

Applies the functions to the places where we would panic or die
`sys::exit()` instead.

Change-Id: I15ca05cd6f99a25a3378518be94110eab416354e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2475
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/read-http.nix

@@ -0,0 +1,17 @@
+{ depot, pkgs, ... }:
+
+let
+
+  # reads a http request (stdin), and writes all headers to stdout, as netencoded dict
+  read-http = depot.users.Profpatsch.writers.rustSimple {
+    name = "read-http";
+    dependencies = [
+      depot.users.Profpatsch.rust-crates.ascii
+      depot.users.Profpatsch.rust-crates.httparse
+      depot.users.Profpatsch.netencode.netencode-rs
+      depot.users.Profpatsch.arglib.netencode.rust
+      depot.users.Profpatsch.execline.exec-helpers
+    ];
+  } (builtins.readFile ./read-http.rs);
+
+in read-http