tests: web: Have the retry-on-error throw on exhausted attempts.

Previously, it'd simply return the error arguments, which would not always
fail a 'test-assert' test, for example.

* gnu/tests/web.scm (retry-on-error): Re-throw the error when the attempts
have been exhausted.

Change-Id: Ic1468d8ba23f0f78377e90d19bccb2878dc56f0e

gnu/tests/web.scm

@@ -83,7 +83,7 @@
            return-value)
           ((#f . error-args)
            (if (>= attempt times)
-               error-args
+               (apply throw error-args)
                (begin
                  (sleep delay)
                  (loop (+ 1 attempt)))))))))