[Feature Request]: Better error reporting than `VasprunXMLValidator` error upon a crash

[Andrew-S-Rosen]

Problem

When a VASP job fails for reasons that Custodian cannot fix, we get the VasprunXMLValidator ValidationError that everyone knows and loves. This is in principle fine, but new users often get confused by this since it's not clear to them what this error means. We should probably return a clearer error message, informing the user that Custodian could not succeed in completing the job and that they should investigate the stderr, OUTCAR, etc.

Migrated from materialsproject/.github#2 where I posted by mistake.

Proposed Solution

Catch the ValidationError and provide a clearer message for the user.