rpc: do not crash when unable to parse JSON

When unable to parse JSON response from the server, properly raise
JSONError not to cause a crash.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/rpc.py

@@ -1078,7 +1078,7 @@ class JSONServerProxy(object):
         try:
             response = json_decode_binary(json.loads(response.decode('ascii')))
         except ValueError as e:
-            raise JSONError(str(e))
+            raise JSONError(error=str(e))
 
         if self.__verbose >= 2:
             root_logger.info(