[rubygems/rubygems] Show error when `rake check_manifest` fails for unknown reason

https://github.com/rubygems/rubygems/commit/98de85c11e

test/rubygems/test_project_sanity.rb

@@ -5,11 +5,28 @@ require "open3"
 
 class TestProjectSanity < Gem::TestCase
   def test_manifest_is_up_to_date
-    pend unless File.exist?(File.expand_path("../../Rakefile", __dir__))
+    pend unless File.exist?("#{root}/Rakefile")
 
     _, status = Open3.capture2e("rake check_manifest")
 
-    assert status.success?, "Expected Manifest.txt to be up to date, but it's not. Run `rake update_manifest` to sync it."
+    unless status.success?
+      original_contents = File.read("#{root}/Manifest.txt")
+
+      # Update the manifest to see if it fixes the problem
+      Open3.capture2e("rake update_manifest")
+
+      out, status = Open3.capture2e("rake check_manifest")
+
+      # If `rake update_manifest` fixed the problem, that was the original
+      # issue, otherwise it was an unknown error, so print the error output
+      if status.success?
+        File.write("#{root}/Manifest.txt", original_contents)
+
+        raise "Expected Manifest.txt to be up to date, but it's not. Run `rake update_manifest` to sync it."
+      else
+        raise "There was an error running `rake check_manifest`: #{out}"
+      end
+    end
   end
 
   def test_require_rubygems_package
@@ -17,4 +34,10 @@ class TestProjectSanity < Gem::TestCase
 
     assert status.success?, err
   end
+
+  private
+
+  def root
+    File.expand_path("../..", __dir__)
+  end
 end