Handle errors gracefully (config parse error, empty namenodes list)

1. If there's config parse error, catch it and print error message. Don't
stop the process, just print message.
2. If namenodes list for HAClient is empty throw exception in the
__init__ method, and print a sane error message.
3. If we find no namenodes in AutoConfigClient __init__ method throw
exception with a sane message.