Temporarily raise the limit of failing Firefox downloads to 14 days.

This should green up most bots during investigation of a
proper fix.


Review-Url: https://codereview.chromium.org/2858023003 .
diff --git a/download_firefox_nightly.py b/download_firefox_nightly.py
index 6523eac..71bfe8d 100755
--- a/download_firefox_nightly.py
+++ b/download_firefox_nightly.py
@@ -94,9 +94,9 @@
     if not fallback_build:
       raise Exception('We failed to download Firefox and we have no builds to '
                       'fall back on; failing...')
-    if age_days > 7:
+    if age_days > 14:
       raise Exception('We have failed to download firefox builds for more '
-                      'than 7 days now: failing so someone looks at it. The '
+                      'than 14 days now: failing so someone looks at it. The '
                       'most recent build we have is %d days old.' % age_days)
     print 'Using %s instead; it is %d days old.' % (fallback_build, age_days)