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).

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EDIT: false alarm, main branch looks ok, it was outdated tmp during my initial test.

[deivid-rodriguez]

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

[simi]

@sarahsehr friendly ping, is there still interest to finish this? Happy to help or take over to fix the one blocking comment in plugin.rb.

[sarahsehr]

@sarahsehr friendly ping, is there still interest to finish this? Happy to help or take over to fix the one blocking comment in plugin.rb.

Thanks for the friendly ping! Coincidentally, I was planning to work on this today. :)

[deivid-rodriguez]

We're almost done with a big rework of how plugins currently work at #6957, I wonder if that could actually fix/improve this problem.

[sarahsehr]

We're almost done with a big rework of how plugins currently work at #6957, I wonder if that could actually fix/improve this problem.

Thanks for pointing this out! I pulled the branch from that PR and was able to reproduce the issue, so unfortunately it doesn't.

[sarahsehr]

@simi I have changed the output to warning and continue the install. The output looks like above (ignore the black line, that's debug output from another gem).

[deivid-rodriguez]

This seems like a strict improvement, let's merge it!

[deivid-rodriguez]

Thanks so much @sarahsehr!