Use a more sensible message for the AbuseReport uniqueness validation

Previously it was "user has already been taken", when really we were
saying the user has already been reported.

app/models/abuse_report.rb

@@ -17,7 +17,7 @@ class AbuseReport < ActiveRecord::Base
   validates :reporter, presence: true
   validates :user, presence: true
   validates :message, presence: true
-  validates :user_id, uniqueness: true
+  validates :user_id, uniqueness: { message: 'has already been reported' }
 
   def remove_user
     user.block

spec/models/abuse_report_spec.rb

@@ -26,7 +26,7 @@ RSpec.describe AbuseReport, type: :model do
     it { is_expected.to validate_presence_of(:reporter) }
     it { is_expected.to validate_presence_of(:user) }
     it { is_expected.to validate_presence_of(:message) }
-    it { is_expected.to validate_uniqueness_of(:user_id) }
+    it { is_expected.to validate_uniqueness_of(:user_id).with_message('has already been reported') }
   end
 
   describe '#remove_user' do