write logs of subexecutions to main log

[bolandka]

see #66

[kba]

Possibilities:

• Use a cloud logging solution and make the full logs available
• Expose the backend log file via the frontend (c.f. infolis/infolis-web@9e636e2), e.g. http://infolis.gesis.org/infolink/backend/icpsr-pdfbox-fixed.csv

[kba]

See https://github.com/infolis/infoLink/tree/loggly for a proof-of-concept for loggly