Use error codes in the parser rather than just outputting human-readable strings

[giovannipizzi]

At the moment, warnings that are detected are simply added as human-readable strings into a 'warnings' list in the parsed output.

Task:

• define which error codes we want to have (this can be done in this issue)
• implement these

[greschd]

@giovannipizzi, is the intent to change this before the 2.0 release, or after?

[giovannipizzi]

I would has said after, but since 2.0 is out, this is now obvious ;-)

I think we can drop py2 support and move on, and do additional improvements for the upcoming release (we needed to have something working with AiiDA 1.0 and both python versions so people in AiiDA 1.0 can use it).

[greschd]

When adding more exit codes, I think it makes sense to follow a convention similar (or equal) to aiidateam/aiida-quantumespresso#296 (comment)