Make method to check a line can be added generic

In the Knapsack voting style, we can't add an investment if its cost is
greater than the money we've got left, but in other voting styles money
might not be the issue.

So we're introducing the term "resources" and adapting the code
accordingly.

app/models/budget/ballot.rb

@@ -64,8 +64,9 @@ class Budget
       @voting_style ||= voting_style_class.new(self)
     end
     delegate :amount_available, :amount_available_info, :amount_spent, :amount_spent_info,
-             :amount_limit_info, :change_vote_info, :enough_money?, :formatted_amount_available,
-             :formatted_amount_limit, :formatted_amount_spent, :voted_info,
+             :amount_limit_info, :change_vote_info, :enough_resources?, :formatted_amount_available,
+             :formatted_amount_limit, :formatted_amount_spent, :not_enough_resources_error,
+             :reason_for_not_being_ballotable, :voted_info,
              to: :voting_style
 
     private

app/models/budget/ballot/line.rb

@@ -10,7 +10,7 @@ class Budget
       validates :ballot_id, :investment_id, :heading_id, :group_id, :budget_id, presence: true
 
       validate :check_selected
-      validate :check_sufficient_funds
+      validate :check_enough_resources
       validate :check_valid_heading
 
       scope :by_investment, ->(investment_id) { where(investment_id: investment_id) }
@@ -18,9 +18,12 @@ class Budget
       before_validation :set_denormalized_ids
       after_save :store_user_heading
 
-      def check_sufficient_funds
+      def check_enough_resources
         ballot.lock!
-        errors.add(:money, "insufficient funds") unless ballot.enough_money?(investment)
+
+        unless ballot.enough_resources?(investment)
+          errors.add(:resources, ballot.not_enough_resources_error)
+        end
       end
 
       def check_valid_heading

app/models/budget/investment.rb

@@ -268,8 +268,9 @@ class Budget
       return :not_selected               unless selected?
       return :no_ballots_allowed         unless budget.balloting?
       return :different_heading_assigned unless ballot.valid_heading?(heading)
-      return :not_enough_money           unless ballot.enough_money?(self)
       return :casted_offline             if ballot.casted_offline?
+
+      ballot.reason_for_not_being_ballotable(self)
     end
 
     def permission_problem(user)

app/models/budget/voting_styles/knapsack.rb

@@ -1,8 +1,16 @@
 class Budget::VotingStyles::Knapsack < Budget::VotingStyles::Base
-  def enough_money?(investment)
+  def enough_resources?(investment)
     investment.price.to_i <= amount_available(investment.heading)
   end
 
+  def reason_for_not_being_ballotable(investment)
+    :not_enough_money unless enough_resources?(investment)
+  end
+
+  def not_enough_resources_error
+    "insufficient funds"
+  end
+
   def amount_available(heading)
     amount_limit(heading) - amount_spent(heading)
   end