Update switch drivers to handle errors

[naved001]

#929 sets up a way for handling errors raised by switches.

• For brocade and dellnos9 switches, their _make_request function log errors. I think we can just modify it to raise a switch error which the networking server will handle.

• For our pexpect drivers, I don't know if we can do much. Maybe catch some pexpect timeout error (when connecting to the switch) and raise SwitchError (or subclass it). That should do it.

@SahilTikale tagging you on this since you are working on two switch drivers right now and you may want to keep this in mind.