lib/readers: factor ErrorReader from multiple sources

lib/readers/error_test.go

@@ -0,0 +1,18 @@
+package readers
+
+import (
+	"testing"
+
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestErrorReader(t *testing.T) {
+	errRead := errors.New("boom")
+	r := ErrorReader{errRead}
+
+	buf := make([]byte, 16)
+	n, err := r.Read(buf)
+	assert.Equal(t, errRead, err)
+	assert.Equal(t, 0, n)
+}