[#925] Better error msg and key_diag now updated

[dvezinet]

Main changes:

• get_dvos_xyz() has more informative error msg

Issues:

Fixes, in devel, issue #925

[pep8speaks]

Hello @dvezinet! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file tofu/data/_class8_vos.py:

Line 993:80: E501 line too long (91 > 79 characters)