Improve error message about logging.

[x-yuri]

simp_le might say:

Challenge validation has failed, see error log.

Is there a log file?

[zenhack]

No, there's not a separate log file -- it just logs to stderr. you can pass --verbose to get more detailed logging.

[x-yuri]

I believe the message is misleading. It can probably raise "Challenge validation has failed", which would print this, and then:

Debugging tips: -v improves output verbosity. Help is available under --help.

Although ideally I'd suggest to rerun with -v only if it wasn't passed. And about --help, it's kind of obvious. In my view, of course.

[zenhack]

Yeah, I agree. I'll change the issue title to reflect that, and we can use this issue to track it.