Add logging to build_scan.py for url retries

R=stip

Review-Url: https://codereview.chromium.org/2235063003
diff --git a/scripts/slave/build_scan.py b/scripts/slave/build_scan.py
index 1561c0d..410bd05 100755
--- a/scripts/slave/build_scan.py
+++ b/scripts/slave/build_scan.py
@@ -38,7 +38,12 @@
         raise
 
       attempts += 1
-      time.sleep(2 ** attempts)
+      time_to_sleep = 2 ** attempts
+      logging.info(
+        "url fetch encountered %s, sleeping for %d seconds and retrying..." % (
+            f, time_to_sleep))
+
+      time.sleep(time_to_sleep)
 
 
 def get_root_json(master_url):