Always return exec command output

Always return exec command output, even if the command invocation returns nonzero. This applies to
exec probes and kubelet RunInContainer calls.

pkg/kubelet/kubelet.go

@@ -2880,11 +2880,10 @@ func (kl *Kubelet) RunInContainer(podFullName string, podUID types.UID, containe
 	var buffer bytes.Buffer
 	output := ioutils.WriteCloserWrapper(&buffer)
 	err = kl.runner.ExecInContainer(container.ID, cmd, nil, output, output, false, nil)
-	if err != nil {
-		return nil, err
-	}
-
-	return buffer.Bytes(), nil
+	// Even if err is non-nil, there still may be output (e.g. the exec wrote to stdout or stderr but
+	// the command returned a nonzero exit code). Therefore, always return the output along with the
+	// error.
+	return buffer.Bytes(), err
 }
 
 // ExecInContainer executes a command in a container, connecting the supplied