allow basic logging for http Get err with RegisterErrLogFn

[qbig]

attempt to improve this :

func (g *Group) load(ctx Context, key string, dest Sink) (value ByteView, destPopulated bool, err error) { // TODO(bradfitz): log the peer's error? keep // log of the past few for /groupcachez? It's // probably boring (normal task movement), so not // worth logging I imagine.

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[qbig]

I signed it!

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CLAs look good, thanks!

[qbig]

updated:)

[qbig]

sorry.. but was there a new review? https://github.com/golang/groupcache/pull/81#pullrequestreview-32262402 didn't show anything

[qbig]

@bradfitz any other parts I could improve on this diff ?