[sl4f][e2e] Also retry on http.ClientException.

It seems like http.<anything> can throw this exception when it can't
figure out the http response and maybe some other problems.

TESTED=Re-ran the test. Hard to test for this particular exception,
though.

Change-Id: I29ed7c5e76379c8cd45edfc1359e03d43dfe2019
diff --git a/src/testing/sl4f/client/lib/src/sl4f_client.dart b/src/testing/sl4f/client/lib/src/sl4f_client.dart
index 48dacfc..bc3d2f46 100644
--- a/src/testing/sl4f/client/lib/src/sl4f_client.dart
+++ b/src/testing/sl4f/client/lib/src/sl4f_client.dart
@@ -168,6 +168,8 @@
         continue;
       } on TimeoutException {
         continue;
+      } on http.ClientException {
+        continue;
       }
       return true;
     }