ActionView::MissingTemplate for partials includes underscore.

Missing partial folder/_partial instead of folder/partial.

Closes #13002.

actionview/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   `ActionView::MissingTemplate` includes underscore when raised for a partial.
+
+    Fixes #13002.
+
+    *Yves Senn*
+
 *   Use `set_backtrace` instead of instance variable `@backtrace` in ActionView exceptions
 
     *Shimpei Makimoto*

actionview/lib/action_view/template/error.rb

@@ -41,6 +41,9 @@ def initialize(paths, path, prefixes, partial, details, *)
         'template'
       end
 
+      if partial && path.present?
+        path = path.sub(%r{([^/]+)$}, "_\\1")
+      end
       searched_paths = prefixes.map { |prefix| [prefix, path].join("/") }
 
       out  = "Missing #{template_type} #{searched_paths.join(", ")} with #{details.inspect}. Searched in:\n"

actionview/test/template/lookup_context_test.rb

@@ -249,15 +249,15 @@ def setup
     e = assert_raise ActionView::MissingTemplate do
       @lookup_context.find("foo", %w(parent child), true)
     end
-    assert_match %r{Missing partial parent/foo, child/foo with .* Searched in:\n  \* "/Path/to/views"\n}, e.message
+    assert_match %r{Missing partial parent/_foo, child/_foo with .* Searched in:\n  \* "/Path/to/views"\n}, e.message
   end
 
   test "if a single prefix is passed as a string and the lookup fails, MissingTemplate accepts it" do
     e = assert_raise ActionView::MissingTemplate do
       details = {:handlers=>[], :formats=>[], :locale=>[]}
       @lookup_context.view_paths.find("foo", "parent", true, details)
     end
-    assert_match %r{Missing partial parent/foo with .* Searched in:\n  \* "/Path/to/views"\n}, e.message
+    assert_match %r{Missing partial parent/_foo with .* Searched in:\n  \* "/Path/to/views"\n}, e.message
   end
 
 end

actionview/test/template/render_test.rb

@@ -63,7 +63,7 @@ def test_render_partial_use_last_prepended_format_for_partials_with_the_same_nam
 
   def test_render_template_with_a_missing_partial_of_another_format
     @view.lookup_context.formats = [:html]
-    assert_raise ActionView::Template::Error, "Missing partial /missing with {:locale=>[:en], :formats=>[:json], :handlers=>[:erb, :builder]}" do
+    assert_raise ActionView::Template::Error, "Missing partial /_missing with {:locale=>[:en], :formats=>[:json], :handlers=>[:erb, :builder]}" do
       @view.render(:template => "with_format", :formats => [:json])
     end
   end
@@ -444,7 +444,7 @@ def test_render_layout_with_a_nested_render_layout_call_using_block_with_render_
 
   def test_render_partial_with_layout_raises_descriptive_error
     e = assert_raises(ActionView::MissingTemplate) { @view.render(partial: 'test/partial', layout: true) }
-    assert_match "Missing partial /true with", e.message
+    assert_match "Missing partial /_true with", e.message
   end
 
   def test_render_with_nested_layout