File to import not found or unreadable: bootstrap (Bootstrap, Rails, Sass error)

I just experienced the same problem

File to import not found or unreadable: bootstrap-sprockets

I tried restarting the development server and running "rails s" again and it worked fine!

I figured out the issue here.

The problem is I am using an :assets group. That was removed in Rails 4.

http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-3-2-to-rails-4-0-gemfile

**4.2 Gemfile**
Rails 4.0 removed the assets group from Gemfile. You'd need to remove that line from your Gemfile when upgrading. 

Once I pulled them out of the assets group, deleted my Gemfile.lock and ran bundle install...everything worked perfectly.

Hope this helps some other poor soul.