Make incompatible_repos consistent accross all controllers

Unify incompatible_repos behavior with other protected methods

app/controllers/import/base_controller.rb

@@ -30,7 +30,7 @@ class Import::BaseController < ApplicationController
   end
 
   def incompatible_repos
-    []
+    raise NotImplementedError
   end
 
   def provider_name

app/controllers/import/fogbugz_controller.rb

@@ -89,6 +89,11 @@ class Import::FogbugzController < Import::BaseController
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
+  override :incompatible_repos
+  def incompatible_repos
+    []
+  end
+
   override :provider_name
   def provider_name
     :fogbugz

app/controllers/import/gitlab_controller.rb

@@ -54,6 +54,11 @@ class Import::GitlabController < Import::BaseController
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
+  override :incompatible_repos
+  def incompatible_repos
+    []
+  end
+
   override :provider_name
   def provider_name
     :gitlab