Workflow exit codes: keep them consistent and adhere to exit code conventions.

[yakutovicha]

As a developer, I would like to see a consistent exit code choice throughout the work chains in the repository. The errors we encounter when running software are typically very similar, so there is no need to recreate them. Also, it is easier to read the exit codes for the user - one might already understand what an error number is.

In AiiDA documentation one can find exit code conventions.

Tasks:

• Agree on the exit code convention and write them to documentation.
• Modify all the exit codes to adhere to the standards