Mailer errors should be more informative

[roscisz]

Traceback (most recent call last):
  File "/usr/local/bin/tensorhive", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.6/dist-packages/tensorhive/__main__.py", line 9, in main
    cli.main()
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1114, in invoke
    return Command.invoke(self, ctx)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/tensorhive/cli.py", line 106, in main
    manager.configure_services_from_config()
  File "/usr/local/lib/python3.6/dist-packages/tensorhive/core/managers/TensorHiveManager.py", line 101, in configure_services_from_config
    services = self.instantiate_services_from_config()
  File "/usr/local/lib/python3.6/dist-packages/tensorhive/core/managers/TensorHiveManager.py", line 85, in instantiate_services_from_config
    email_sending_handler = ProtectionHandler(behaviour=EmailSendingBehaviour())
  File "/usr/local/lib/python3.6/dist-packages/tensorhive/core/violation_handlers/EmailSendingBehaviour.py", line 44, in __init__
    self._test_smtp_configuration()
  File "/usr/local/lib/python3.6/dist-packages/tensorhive/core/violation_handlers/EmailSendingBehaviour.py", line 115, in _test_smtp_configuration
    self.mailer.connect(login=MAILBOT.SMTP_LOGIN, password=MAILBOT.SMTP_PASSWORD)
  File "/usr/local/lib/python3.6/dist-packages/tensorhive/core/utils/mailer.py", line 86, in connect
    self.server = smtplib.SMTP(self.smtp_server, self.smtp_port)
  File "/usr/lib/python3.6/smtplib.py", line 251, in __init__
    (code, msg) = self.connect(host, port)
  File "/usr/lib/python3.6/smtplib.py", line 336, in connect
    self.sock = self._get_socket(host, port, self.timeout)
  File "/usr/lib/python3.6/smtplib.py", line 307, in _get_socket
    self.source_address)
  File "/usr/lib/python3.6/socket.py", line 704, in create_connection
    for res in getaddrinfo(host, port, 0, SOCK_STREAM):
  File "/usr/lib/python3.6/socket.py", line 745, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -2] Name or service not known