cli/command/context: context ls: always show current context

if a context is set (e.g. through DOCKER_CONTEXT or the CLI config file), but
wasn't found, then a "stub" context is added, including an error message that
the context doesn't exist.

    DOCKER_CONTEXT=nosuchcontext docker context ls
    NAME              DESCRIPTION                               DOCKER ENDPOINT               ERROR
    default           Current DOCKER_HOST based configuration   unix:///var/run/docker.sock
    nosuchcontext *                                                                           context "nosuchcontext": context not found: …

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/context/list_test.go

@@ -39,3 +39,11 @@ func TestListQuiet(t *testing.T) {
 	assert.NilError(t, runList(cli, &listOptions{quiet: true}))
 	golden.Assert(t, cli.OutBuffer().String(), "quiet-list.golden")
 }
+
+func TestListError(t *testing.T) {
+	cli := makeFakeCli(t)
+	cli.SetCurrentContext("nosuchcontext")
+	cli.OutBuffer().Reset()
+	assert.NilError(t, runList(cli, &listOptions{}))
+	golden.Assert(t, cli.OutBuffer().String(), "list-with-error.golden")
+}