Improve error message when config file cannot be read (#13)
Improve error message when config file cannot be read (#13)

If there's an error reading the config file, the process panics without any
indication of the cause of the panic.

To fix this, `and_then` is used when reading the config file so that "failed to
read config" is included in the panic.

linkerd/linkerd-tcp,linkerd/linkerd-tcp