Improve error message from scraper

[simonswine]

This also includes the response body of the non 2xx response, so it is clearer what is wrong with the scrape.

Before:

$ curl -s localhost:4100/targets | jq '.activeTargets[] | select(.lastError!="") | .lastError' | sort -u
"server returned HTTP status 500 Internal Server Error"

After:

$ curl -s localhost:4100/targets | jq '.activeTargets[] | select(.lastError!="") | .lastError' | sort -u
"server returned HTTP status (500) Could not enable CPU profiling: cpu profiling already in use"

[cyriltovena]

LGTM