[autotest] Fix error message in autotest.py

TEST=Threw and exception.
BUG=None

Change-Id: I548d1afb936ad9f6e6e2580833c47f9dbb1f4aa6
Reviewed-on: https://chromium-review.googlesource.com/170569
Tested-by: Prashanth B <[email protected]>
Commit-Queue: Prashanth B <[email protected]>
Reviewed-by: Aviv Keshet <[email protected]>
diff --git a/server/autotest.py b/server/autotest.py
index 2199ed1..2cac5b1 100644
--- a/server/autotest.py
+++ b/server/autotest.py
@@ -647,7 +647,7 @@
                 new_boot_id = self.host.get_boot_id(timeout=60)
             except Exception as e:
                 msg += ('DUT is pingable but not SSHable, it most likely'
-                        ' sporadically rebooted during testing. %s\n', str(e))
+                        ' sporadically rebooted during testing. %s\n' % str(e))
                 raise AutotestDeviceNotSSHable(msg)
             else:
                 if new_boot_id != old_boot_id: