[hadoop] Protect against non defined server_exc attribute in WebHdfsException

Problem: when first time login with dt_login, an exception occurs and prevents user from proceeding.
To reproduce: set up Hue with hue.ini such that webhdfs_url is localhost:port/webhdfs/v1 instead of hostname:port/webhdfs/v1 .
This will get an ('Connection aborted.', error(111, 'Connection refused')) error when trying to connect to webhdfs. During first time login, the ensure_home_directory function is called and the attached error occur.

Patch attached.