Make ensure methods re-usable across actions

Previously these were only used on edit actions which was fine because
the `render_edit_with_errors` method is able to accommodate both the `update_amount` and `invalidate` actions. However, I want to re-use the `ensure_amount` method inside of the new/create flow.

Since new/create flow would need to render a different template (eg.
`render_new_with_errors`) I have altered the ensure method to accept a
block instead of being hard-coded to `render_edit_with_errors`.

admin/app/controllers/solidus_admin/store_credits_controller.rb

@@ -35,8 +35,8 @@ def edit_amount
     end
 
     def update_amount
-      return unless ensure_amount
-      return unless ensure_store_credit_reason
+      return unless ensure_amount { render_edit_with_errors }
+      return unless ensure_store_credit_reason { render_edit_with_errors }
 
       if @store_credit.update_amount(permitted_store_credit_params[:amount], @store_credit_reason, spree_current_user)
         respond_to do |format|
@@ -100,7 +100,7 @@ def edit_validity
     end
 
     def invalidate
-      return unless ensure_store_credit_reason
+      return unless ensure_store_credit_reason { render_edit_with_errors }
 
       if @store_credit.invalidate(@store_credit_reason, spree_current_user)
         flash[:notice] = t('.success')
@@ -171,7 +171,7 @@ def render_edit_with_errors
     def ensure_amount
       if permitted_store_credit_params[:amount].blank?
         @store_credit.errors.add(:amount, :greater_than, count: 0, value: permitted_store_credit_params[:amount])
-        render_edit_with_errors
+        yield if block_given? # Block is for error template rendering on a per-action basis so this can be re-used.
         return false
       end
       true
@@ -182,7 +182,7 @@ def ensure_store_credit_reason
 
       if @store_credit_reason.blank?
         @store_credit.errors.add(:store_credit_reason_id, "Store Credit reason must be provided")
-        render_edit_with_errors
+        yield if block_given? # Block is for error template rendering on a per-action basis so this can be re-used.
         return false
       end
       true