Exit with access error if rebase container cannot access previous app…

… image or run image

- This prevents opaque errors where the rebase would fail trying to read labels on a inaccessible image:
```
ERROR: rebase working image: rebase: could not find base layer in image
```

cmd/rebase/main.go

@@ -9,6 +9,7 @@ import (
 	"github.com/buildpacks/imgutil/remote"
 	"github.com/buildpacks/lifecycle"
 	"github.com/buildpacks/lifecycle/cmd"
+	"github.com/pkg/errors"
 
 	"github.com/pivotal/kpack/pkg/dockercreds"
 	"github.com/pivotal/kpack/pkg/flaghelpers"
@@ -74,11 +75,19 @@ func rebase(tags []string, logger *log.Logger) error {
 		return err
 	}
 
+	if !appImage.Found() {
+		return errors.Errorf("could not access previous image: %s", *lastBuiltImage)
+	}
+
 	newBaseImage, err := remote.NewImage(*runImage, keychain, remote.FromBaseImage(*runImage))
 	if err != nil {
 		return err
 	}
 
+	if !newBaseImage.Found() {
+		return errors.Errorf("could not access run image: %s", *runImage)
+	}
+
 	rebaser := lifecycle.Rebaser{
 		Logger: cmd.Logger,
 	}