[errors] Greatly simplify common/errors package.

dm/appengine/mutate/add_deps.go

Index: dm/appengine/mutate/add_deps.go
diff --git a/dm/appengine/mutate/add_deps.go b/dm/appengine/mutate/add_deps.go
index 0645ffa03e3d28b568ec25c095f140eec7fcb669..5f34cec4d0898de96b1f1ee72857753627c9c165 100644
--- a/dm/appengine/mutate/add_deps.go
+++ b/dm/appengine/mutate/add_deps.go
@@ -51,8 +51,7 @@ func (a *AddDeps) RollForward(c context.Context) (muts []tumble.Mutation, err er
 	}
 
 	fwdDeps, err := filterExisting(c, model.FwdDepsFromList(c, a.Auth.Id.AttemptID(), a.Deps))
-	err = errors.Annotate(err).Tag(grpcutil.Tag.With(codes.Internal)).
-		Reason("while filtering deps").Err()
+	err = errors.Annotate(err, "while filtering deps").Tag(grpcutil.Tag.With(codes.Internal)).Err()
 	if err != nil || len(fwdDeps) == 0 {
 		return
 	}
@@ -66,8 +65,7 @@ func (a *AddDeps) RollForward(c context.Context) (muts []tumble.Mutation, err er
 	}
 
 	if err = ds.Put(c, fwdDeps, atmpt, ex); err != nil {
-		err = errors.Annotate(err).Tag(grpcutil.Tag.With(codes.Internal)).
-			Reason("putting stuff").Err()
+		err = errors.Annotate(err, "putting stuff").Tag(grpcutil.Tag.With(codes.Internal)).Err()
 		return
 	}