[errors] Greatly simplify common/errors package.

luci_config/server/cfgclient/backend/authority.go

Index: luci_config/server/cfgclient/backend/authority.go
diff --git a/luci_config/server/cfgclient/backend/authority.go b/luci_config/server/cfgclient/backend/authority.go
index 819fd873f17f6da50fdaf6fdb8800783178dea7c..a1b74a8b51c4305fd41aa68140d0a98931c3d1b7 100644
--- a/luci_config/server/cfgclient/backend/authority.go
+++ b/luci_config/server/cfgclient/backend/authority.go
@@ -52,7 +52,7 @@ func (a Authority) MarshalJSON() ([]byte, error) {
 	case AsUser:
 		return asUserJSON, nil
 	default:
-		return nil, errors.Reason("unknown authority: %(auth)v").D("auth", a).Err()
+		return nil, errors.Reason("unknown authority: %v", a).Err()
 	}
 }
 
@@ -66,7 +66,7 @@ func (a *Authority) UnmarshalJSON(d []byte) error {
 	case bytes.Equal(d, asUserJSON):
 		*a = AsUser
 	default:
-		return errors.Reason("unknown authority JSON value: %(auth)v").D("auth", d).Err()
+		return errors.Reason("unknown authority JSON value: %v", d).Err()
 	}
 	return nil
 }