give a better error message for misspelled helpers

See comment in this patch for the rationale.

References #16468

actionpack/lib/abstract_controller/helpers.rb

@@ -150,7 +150,17 @@ module AbstractController
             rescue LoadError => e
               raise AbstractController::Helpers::MissingHelperError.new(e, file_name)
             end
-            file_name.camelize.constantize
+
+            mod_name = file_name.camelize
+            begin
+              mod_name.constantize
+            rescue LoadError
+              # dependencies.rb gives a similar error message but its wording is
+              # not as clear because it mentions autoloading. To the user all it
+              # matters is that a helper module couldn't be loaded, autoloading
+              # is an internal mechanism that should not leak.
+              raise NameError, "Couldn't find #{mod_name}, expected it to be defined in helpers/#{file_name}.rb"
+            end
           when Module
             arg
           else

actionpack/test/controller/helper_test.rb

@@ -60,6 +60,12 @@ class HelpersPathsController < ActionController::Base
   end
 end
 
+class HelpersTypoController < ActionController::Base
+  path = File.expand_path('../../fixtures/helpers_typo', __FILE__)
+  $:.unshift(path)
+  self.helpers_path = path
+end
+
 module LocalAbcHelper
   def a() end
   def b() end
@@ -82,6 +88,22 @@ class HelperPathsTest < ActiveSupport::TestCase
   end
 end
 
+class HelpersTypoControllerTest < ActiveSupport::TestCase
+  def setup
+    @autoload_paths = ActiveSupport::Dependencies.autoload_paths
+    ActiveSupport::Dependencies.autoload_paths = Array(HelpersTypoController.helpers_path)
+  end
+
+  def test_helper_typo_error_message
+    e = assert_raise(NameError) { HelpersTypoController.helper 'admin/users' }
+    assert_equal "Couldn't find Admin::UsersHelper, expected it to be defined in helpers/admin/users_helper.rb", e.message
+  end
+
+  def teardown
+    ActiveSupport::Dependencies.autoload_paths = @autoload_paths
+  end
+end
+
 class HelperTest < ActiveSupport::TestCase
   class TestController < ActionController::Base
     attr_accessor :delegate_attr

actionpack/test/fixtures/helpers_typo/admin/users_helper.rb

@@ -0,0 +1,5 @@
+module Admin
+  module UsersHelpeR
+  end
+end
+