How to throw and test errors in Emacs Lisp?

(defun foo (x)
  "Return x.  If x < 0, throw an error."
  (if (< x 0)
      (error "%s is negative" x)
    x))

(ert-deftest foo-test ()
  (should (= (foo 10) 10))
  (should-error (foo -1)))

(foo -1)