Fix list error messages (mastodon#33082)

app/models/list_account.rb

@@ -25,17 +25,16 @@ class ListAccount < ApplicationRecord
   private
 
   def set_follow
-    self.follow = Follow.find_by!(account_id: list.account_id, target_account_id: account.id)
-  rescue ActiveRecord::RecordNotFound
-    self.follow_request = FollowRequest.find_by!(account_id: list.account_id, target_account_id: account.id)
+    self.follow = Follow.find_by(account_id: list.account_id, target_account_id: account.id)
+    self.follow_request = FollowRequest.find_by(account_id: list.account_id, target_account_id: account.id) if follow.nil?
   end
 
   def validate_relationship
-    return if list.account_id == account_id
+    return if list_owner_account_is_account?
 
-    errors.add(:account_id, 'follow relationship missing') if follow_id.nil? && follow_request_id.nil?
-    errors.add(:follow, 'mismatched accounts') if follow_id.present? && follow.target_account_id != account_id
-    errors.add(:follow_request, 'mismatched accounts') if follow_request_id.present? && follow_request.target_account_id != account_id
+    errors.add(:account_id, :must_be_following) if follow_id.nil? && follow_request_id.nil?
+    errors.add(:follow, :invalid) if follow_id.present? && follow.target_account_id != account_id
+    errors.add(:follow_request, :invalid) if follow_request_id.present? && follow_request.target_account_id != account_id
   end
 
   def list_owner_account_is_account?

config/locales/activerecord.en.yml

@@ -39,6 +39,11 @@ en:
           attributes:
             data:
               malformed: is malformed
+        list_account:
+          attributes:
+            account_id:
+              taken: is already on the list
+          must_be_following: must be a followed account
         status:
           attributes:
             reblog:

spec/requests/api/v1/lists/accounts_spec.rb

@@ -95,7 +95,7 @@
       it 'does not add the account to the list', :aggregate_failures do
         subject
 
-        expect(response).to have_http_status(404)
+        expect(response).to have_http_status(422)
         expect(response.content_type)
           .to start_with('application/json')
         expect(list.accounts).to_not include(bob)