Add more description to failed validation messages

[its02003]

While the doc and example depths are still being built out, it would be helpful to return as much information in error messages as possible, for instance:

In the oslo spec project there isn't much depth around what options there are for fields like the SLO.Spec.TimeWindows[].Unit field, though the validator code does a nice job of validating those items here.

It would be useful in this area and others to return useful information like what options must be specified in the error messages, so that sorry suckers like yours truly don't have to slap on the diving equipment and find the answer for ourselves :)

Current behavior: Error: Key: 'SLO.Spec.TimeWindows[0].Unit' Error:Field validation for 'Unit' failed on the 'oneof' tag
Expected behavior: Error: Key: 'SLO.Spec.TimeWindows[0].Unit' Error:Field validation for 'Unit' failed on the 'oneof' tag [Second Quarter Month Week Day]

[its02003]

I am going to open an issue on the validator project for this

[its02003]

@ian-bartholomew I have a PR I am finalizing for the validator project to hopefully fix this

[ian-bartholomew]

@its02003 amazing, thank you!