lib/readers: factor ErrorReader from multiple sources

lib/readers/error.go

@@ -0,0 +1,11 @@
+package readers
+
+// ErrorReader wraps an error to return on Read
+type ErrorReader struct {
+	Err error
+}
+
+// Read always returns the error
+func (er ErrorReader) Read(p []byte) (n int, err error) {
+	return 0, er.Err
+}