Add useful error message for plugin load

[sarahsehr]

What was the end-user or developer problem that led to this PR?

When starting to work on my first issue, I found that rake setup failed for me. This was due to having previously installed a plugin, whose files had since been deleted (presumably when I uninstalled that version of Ruby).

#7636

What is your fix for the problem, implemented in this PR?

Check to see whether each load path for the plugin is a directory. If it is not, notify the user in a friendly message which plugin in the problem and how to resolve the issue.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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[simi]

Hello @sarahsehr. Code looks good, but some specs are failing, which seems not really related to your change, since I can see the same failure on main branch. I'm looking into. 👀

EDIT: false alarm, main branch looks ok, it was outdated tmp during my initial test.

[simi]

Have you considered just warn in here and skip plugin + continue? 🤔

[simi]

@sarahsehr this seems to fails the test (and breaks some test gem installation at the same time). add_to_load_path is defined at

rubygems/lib/rubygems.rb

Lines 576 to 584 in c1933bf

	##
	# Add a list of paths to the $LOAD_PATH at the proper place.
	def self.add_to_load_path(*paths)
	@activated_gem_paths = activated_gem_paths + paths.size
	# gem directories must come after -I and ENV['RUBYLIB']
	$LOAD_PATH.insert(Gem.load_path_insert_index, *paths)
	end

and it expects multiple arguments. To achieve this by passing paths array, you need to add splat (*) operator. Changing this to Gem.add_to_load_path(*paths) should fix this. 🙏

[deivid-rodriguez]

Since this a bundle install run after all, should we automatically reinstall any missing plugins if necessary, instead of erroring out?