discover: Print more information on failure

[timbertson]

Just putting this here in case it's useful. Without peeking into discover.ml I'd have no idea how to get at the logs when configure fails. This tells you to set DEBUG=1 if the logs are going to be deleted, and once you have done that it'll print the location of the log on failure.

[aantron]

Thanks. The shadowing of exit is probably not that great, but we will have to take a look at improving discover.ml anyway in the future, and we can pick a better name then.