[Feature] Error handling

[ezpzbz]

Once we are done with addressing #6, we can benefit from nice error handlers implemented in custodian to have similar mechanisms here. It can be done either by:

• Adding custodian as a dependency to out package
• or taking the codes and solutions from them (of course with proper citing and acknowledgment)

[ezpzbz]

I've tried different ways to implement the error handlers (keeping in mind to avoid changes in plugin), however, it is inevitable at the end. The final solution that I came up with is as follows:

• [x] Get the list of possible errors from cusodian
• [x] Define different exit codes for them in aiida-vasp plugin.
• [x] Parse errors in workchain: these can be parsed from _scheduler.stdout, _scheduler.stderr, and vasprun.xml
• [x] Connect parsed errors to relevant exit codes. It needs to be done in VaspBaseWorkChain.
• [x] Call proper handler in VaspBaseWorkChain to fix the issue.