Fixed a missing import in exchange_rate.py that caused program hangs and wallet corruption

Signed-off-by: John Miguel Villar Zavatti <johnvillarzavatti@gmail.com>

plugins/exchange_rate.py

@@ -8,6 +8,7 @@ import json
 import threading
 import time
 import re
+from ssl import SSLError
 from decimal import Decimal
 from electrum.plugins import BasePlugin, hook
 from electrum.i18n import _