Improve handling of unknown errors in the aurora client.

Instead of dumping the stack on the user's terminal, or
absorbing the error and generating a brief error
message, the client now writes detailed information about
the error is written into an error log file, and the
user is given a clean error message referring them to that
file for details.

Bugs closed: aurora-779

Reviewed at https://reviews.apache.org/r/26328/