Log: tolerate missing rotated log files on startup

Recursive _rotate() may have already moved or unlinked the source path
by the time we try to rename it (also tolerates concurrent logrotate
runs from /etc/cron.reboot). Catch FileNotFoundError instead of
crashing bbctrl on startup.

src/py/bbctrl/Log.py

@@ -182,4 +182,11 @@ class Log(object):
             if n == 16: os.unlink(fullpath)
             else: self._rotate(path, nextN)
 
-            os.rename(fullpath, '%s.%d' % (path, nextN))
+            # The recursive call may have unlinked or rotated this
+            # path; tolerate a missing source rather than crashing
+            # bbctrl on startup. This also tolerates concurrent
+            # logrotate runs from /etc/cron.reboot.
+            try:
+                os.rename(fullpath, '%s.%d' % (path, nextN))
+            except FileNotFoundError:
+                pass