http.manage: Avoid overwriting an existing file with --export-ca.

caucase/http.py

@@ -1142,6 +1142,8 @@ def manage(argv=None, stdout=sys.stdout):
       file=stdout,
     )
   if args.export_ca is not None:
+    if os.path.exists(args.export_ca):
+      parser.error('file exists: %r' % (args.export_ca, ))
     while True:
       passphrase = getBytePass(
         'CA export passphrase: ',