Add error messages for repeated follows and unfollows

lib/pleroma/user.ex

@@ -31,22 +31,31 @@ defmodule Pleroma.User do
 
   def follow(%User{} = follower, %User{} = followed) do
     ap_followers = User.ap_followers(followed)
-    following = [ap_followers | follower.following]
-    |> Enum.uniq
+    if following?(follower, followed) do
+      { :error,
+        "Could not follow user: #{followed.nickname} is already on your list." }
+    else
+      following = [ap_followers | follower.following]
+      |> Enum.uniq
 
-    follower
-    |> follow_changeset(%{following: following})
-    |> Repo.update
+      follower
+      |> follow_changeset(%{following: following})
+      |> Repo.update
+    end
   end
 
   def unfollow(%User{} = follower, %User{} = followed) do
     ap_followers = User.ap_followers(followed)
-    following = follower.following
-    |> List.delete(ap_followers)
+    if following?(follower, followed) do
+      following = follower.following
+      |> List.delete(ap_followers)
 
-    follower
-    |> follow_changeset(%{following: following})
-    |> Repo.update
+      follower
+      |> follow_changeset(%{following: following})
+      |> Repo.update
+    else
+      { :error, "Not subscribed!" }
+    end
   end
 
   def following?(%User{} = follower, %User{} = followed) do