Make it easier to find the cause of CRASH runs.

[esbena]

As seen in #12, it is not clear from stdout why a tool crashed during a run.
While this is intended behaviour that greatly limits the output when doing hundreds of runs at once, it is not useful for understanding the cause of the crash.
To remedy this, we could do several things:

• add a --debug or --verbose flag to bin/cli run to show more output
• store log files for each run, and show their paths at the end of all runs
• show stderr by default for singular runs like the one in How do I fix Error: spawn UNKNOWN with node.js 15.0.0 on windows 10? #12
• ...