Sage 8.5.3 -- Errors after Removing then Re-installing Bootstrap

Hi,

I had the default Bootstrap “v4.0.0-beta” installed, and working.

I did this:
bower uninstall bootstrap --save

Then I did this:
bower install bootstrap --save

Here’s the output of the install operation.

Then, now if I try to run gulp, I get these errors.

How do I fix this. Why does this happen?

I have a full folder backup of the theme before I did this (except node_modules).