Cutadapt hangs after raising error

[marcelm]

The error handling needs to be improved when running on multiple cores.

Motivated by the attempted fix for #525 in commit 7aa04206196c29f4988f57ccdc01ad89f978f8b3, which does not quite work.

• An error in the ReaderProcess needs to be correctly propagated to the main process.
• Currently, when there is an error (file not found, for example), then we get a traceback

Read also https://www.cloudcity.io/blog/2019/02/27/things-i-wish-they-told-me-about-multiprocessing-in-python/ once more.