Ruler API should not accept invalid rules files

[cstyan]

The Ruler API that allows users to write rules files to Cortex should return an error to users who try to upload invalid rules files, for example a rule group with no name.

cc @gotjosh

[s-h-a-d-o-w]

Coincidentally, @gotjosh and I spoke about this just before.

To provide some more details - this is the end point: https://cortexmetrics.io/docs/apis/#set-rule-group

And here a collection of cases that should return an error:

• Empty payload
• No rule group name
• No rules

Possibly also:

• Props that don't exist on the schema. Reason being: A user might've made a typo when trying to specify interval.

[s-h-a-d-o-w]

Just noticed that this issue overlaps with this one.

[cstyan]

We can reuse promtool check rules <files> for this. I'll have to make some changes upstream so that that check functionality is available as an importable function first though.