Return sink write errors from every handler (closes #22)

The console and JSON handlers threw away the error from their write to
stdout, so a lost log line looked delivered. They now return it,
wrapped. The webhook handler also returns an error for a status outside
2xx, and no longer follows redirects, which could resend the request
without the record. The multiplex handler passes the record to every
handler and returns their errors joined with errors.Join instead of
stopping at the first.

Both stdout handlers gain an unexported writer, nil meaning os.Stdout at
write time, so tests can supply a sink that fails. The README says what
each handler returns and that the slog.Logger methods discard a
handler's error.

Model: opus-5-5

README.md

@@ -25,6 +25,9 @@ Released v1.0.0 2024-06-14.  Works as intended.  No known bugs.
     in console output they are appended as `key=value` pairs, with grouped
     keys written as `group.key=value`. See
     [Attribute output](#attribute-output) for the details worth knowing
+-   reports a record that could not be delivered as an error from the
+    handler's `Handle` method. See
+    [When delivery fails](#when-delivery-fails) for how to receive it
 
 ## Planned Features
 
@@ -104,6 +107,28 @@ ignored, an empty group is elided along with its key, a group with an
 empty key is inlined into its parent, and `WithGroup("")` returns the
 handler unchanged.
 
+## When delivery fails
+
+Every handler returns an error from `Handle` when it could not deliver
+the record:
+
+-   `ConsoleHandler` and `JSONHandler` return the error from their write to
+    stdout, wrapped, so `errors.Is` still matches the original
+-   `WebhookHandler` returns an error when the request fails, and also when
+    the server answers with a status outside 2xx, a redirect included,
+    since it does not follow redirects
+-   `MultiplexHandler`, which simplelog installs as the default, passes the
+    record to every handler it holds even after one of them fails, then
+    returns all their errors joined with `errors.Join` (nil if none failed)
+
+`slog.Info`, `slog.Error` and the other `slog.Logger` methods throw that
+error away. That is how `log/slog` works, and simplelog cannot change it.
+To find out whether a record was delivered, build a `slog.Record` and pass
+it to the handler yourself, for example
+`slog.Default().Handler().Handle(ctx, record)`, then check the error it
+returns. A record passed to `Handle` directly gets the wrong file and
+line in console output.
+
 ## Entrypoints
 
 This repository adheres to the