Ruler: write meaningful logs when remote-write is disabled or is misconfigured

[dannykopping]

What this PR does / why we need it:
This PR adds more meaningful logs when recording rules are present, but remote-write is either disabled or misconfigured.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
The previous log messages looked like this:

... msg="Rule evaluation result discarded" err=unimplemented sample="{__name__=\"test\"} => 2 @[1628673147231]"

And now look like this:

... msg="Rule evaluation result discarded" err="remote-write disabled" sample="{__name__=\"test\"} => 2 @[1628673202231]"

Checklist

• Documentation added
• Tests updated