How to stub raising exceptions in rspec

exception_spec.rb

class Foo
  class << self
    def exec
      something
    rescue => ex
      ex.message
    end
    
    def something
      "something"
    end

  end
end


describe Foo do
  describe ".exec()" do
       
    context "when error occured" do            
      before do
        Foo.stub(:something).and_raise(StandardError.new("error"))
      end

      subject(:msg) { Foo.exec }
      it "returns its error message" do        
        expect(msg).to eq("error")
      end
    end
    
  end
end