Improve RunVaspCustodian to store standard error output

[mkhorton]

Often XMLValidationErrors are actually the result of some other issue that's been reported in std_err.txt, it would be useful to include this in the stored_data also for easier inspection.

[janosh]

Came here to suggest the same thing. :)

[janosh]

Actually, can't this be done with Fireworks trackers? Something like

from atomate.vasp.powerups import add_trackers

wf = add_trackers(wf, ["std_err.txt"], nlines=50)

[mkhorton]

Yes, trackers are designed for this purpose in general. I still think XMLValidationError could be improved however, because it essentially contains zero information right now.