Run stringer on codes.Code

This makes error codes easier to read:
fmt.Sprint(codes.OK) == "OK" instead of "0"

codes/code_string.go

@@ -0,0 +1,16 @@
+// generated by stringer -type=Code; DO NOT EDIT
+
+package codes
+
+import "fmt"
+
+const _Code_name = "OKCanceledUnknownInvalidArgumentDeadlineExceededNotFoundAlreadyExistsPermissionDeniedResourceExhaustedFailedPreconditionAbortedOutOfRangeUnimplementedInternalUnavailableDataLossUnauthenticated"
+
+var _Code_index = [...]uint8{0, 2, 10, 17, 32, 48, 56, 69, 85, 102, 120, 127, 137, 150, 158, 169, 177, 192}
+
+func (i Code) String() string {
+	if i+1 >= Code(len(_Code_index)) {
+		return fmt.Sprintf("Code(%d)", i)
+	}
+	return _Code_name[_Code_index[i]:_Code_index[i+1]]
+}