Make error messages more readable

[contramundum53]

Currently, debugging subprocesses run by kurobako is rather painful.

There can be multiple possible improvements.

• We could show that "During study <study setting>, subprocess <id> finished with an error <error_code>" instead of "kurobako received an empty line".
• We could capture stderr messages of the subprocesses, and only show that of the failing subprocess. This prevents the important error message from being buried in other logs running in parallel.