Fix: Throw error if plugins from config is not an array.

[calvinf]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[X] Other, please explain:

I'm throwing an Error if the plugins set doesn't match expected input type. This isn't a bug necessarily, but it's something I've ran into multiple times on multiple projects where I initially tried to pass in a string (with one plugin name) instead of an array.

What changes did you make? (Give an overview)

I'm throwing an Error with a detailed error message if the user has a config with plugins as anything other than an array (which is expected).

Is there anything you'd like reviewers to focus on?

I have both a debug(/* ... */) statement and throwing an error. I'm not sure if that's the expected convention, but I tried to do similar to what's happening in other parts of the same plugins.js file. Build passes and tests pass.

[eslintbot]

Thanks for the pull request, @calvinf! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[eslintbot]

LGTM

[calvinf]

Updated commit message to match guidelines.

[not-an-aardvark]

Looks good to me, thanks!