Make flux logs more lenient

UX changes:

- Only print an error when a pod doesn't have a matching container
  instead of exiting early.
- Return a non-zero status code when no pod is found at all.

Details:

In certain situations there might be 3rd-party pods running in the
Flux namespace that cause the command to fail streaming logs, e.g.
when they have multiple containers but none of them is called
`manager` (which all Flux-maintained pods do). An example of such a
situation is when Flux is installed with the 3rd-party Flux extension
on AKS.

The `logs` command is now more forgiving and merely logs an error in
these situations instead of completely bailing out. It still returns a
non-zero exit code.

For the parallel log streaming with `-f` the code is now a little more
complex so that errors are now written to stderr in parallel with all
other logs written to stdout. That's what `asyncCopy` is for.

refs #3944

Signed-off-by: Max Jonas Werner <mail@makk.es>

cmd/flux/main_test.go

@@ -392,6 +392,10 @@ func resetCmdArgs() {
 	alertProviderArgs = alertProviderFlags{}
 	bootstrapArgs = NewBootstrapFlags()
 	bServerArgs = bServerFlags{}
+	logsArgs = logsFlags{
+		tail:          -1,
+		fluxNamespace: rootArgs.defaults.Namespace,
+	}
 	buildKsArgs = buildKsFlags{}
 	checkArgs = checkFlags{}
 	createArgs = createFlags{}