progress: tweak progress error reporting

This commit adds catpure of os.Std{out,err} when running osbuild so
that we capture the error log and can display it as part of
an error from osbuild, e.g. when osbuild itself crashes.

This gives us more accurate error reporting if anything fails
during the osbuild building.

bib/pkg/progress/progress_test.go

@@ -3,6 +3,8 @@ package progress_test
 import (
 	"bytes"
 	"fmt"
+	"os"
+	"path/filepath"
 	"reflect"
 	"testing"
 
@@ -130,3 +132,27 @@ func TestProgressNewAutoselect(t *testing.T) {
 		assert.Equal(t, reflect.TypeOf(pb), reflect.TypeOf(tc.expected), fmt.Sprintf("[%v] %T not the expected %T", tc.onTerm, pb, tc.expected))
 	}
 }
+
+func makeFakeOsbuild(t *testing.T, content string) string {
+	p := filepath.Join(t.TempDir(), "fake-osbuild")
+	err := os.WriteFile(p, []byte("#!/bin/sh\n"+content), 0755)
+	assert.NoError(t, err)
+	return p
+}
+
+func TestRunOSBuildWithProgress(t *testing.T) {
+	restore := progress.MockOsbuildCmd(makeFakeOsbuild(t, `echo osbuild-stdout-output
+>&2 echo osbuild-stderr-output
+exit 112
+`))
+	defer restore()
+
+	pbar, err := progress.New("debug")
+	assert.NoError(t, err)
+	err = progress.RunOSBuild(pbar, []byte(`{"fake":"manifest"}`), "", "", nil, nil)
+	assert.EqualError(t, err, `error running osbuild: exit status 112
+Output:
+osbuild-stdout-output
+osbuild-stderr-output
+`)
+}