npm: relayReadFailure assigns HTTPDetail.ResponseBody unscrubbed, so a library consumer reads the raw upstream body

relayReadFailure scrubs one of its two user-controlled fields and not the other.

Raised in review of !263 (merged). Line references are pinned at a904a28.

RequestURL goes through redact.RedactURLUserinfo. ResponseBody is then assigned raw. The same function argues for scrubbing on the grounds that the field is importable, which applies to both.

Both renderers scrub on output, so the exposure reaches library consumers only.

Not introduced by !263 (merged). The assignment came from 70bd7fa, the Steps 16-19 batch (!261 (merged)).