Errors during plugin require() are masked

[eemeli]

When a plugin throws an error when it's required, postcss-cli always prints Plugin Error: Cannot find module '${plugin}' rather than reporting the actual error.

I spent a little while today debugging because of this, as I was getting the above error due to cssnano throwing a BrowserslistError due to a bad config. That root cause would have been significantly easier to figure out if the original error had not been masked.

[RyanZim]

Good point; PR welcome to include the actual error in the error message.