Fixes #967 - In addition with patch for #960, ensure that error logging with not crash

As in python3 errno can be "None" with system raised exceptions, ensure
that this case will not make s3cmd crash just for an error log output.

Author: fviard

s3cmd

@@ -1349,7 +1349,7 @@ def cmd_sync_remote2local(args):
                     error(u"Download of '%s' failed (Reason: No space left)" % file)
                     allow_partial = False
                 else:
-                    error(u"Download of '%s' failed (Reason: Unknown OsError %d)" % (file, exc.errno))
+                    error(u"Download of '%s' failed (Reason: Unknown OsError %d)" % (file, exc.errno or 0))
                     allow_partial = False
 
                 try: