cmd/pprof: restore printing descriptive errors from net/http/pprof endpoints

cmd/pprof: restore printing descriptive errors from net/http/pprof endpoints

Restores functionality added in https://golang.org/cl/35564/ which was
lost in https://golang.org/cl/36798/ by the addition of the custom
fetcher to src/cmd/pprof/pprof.go. The custom fetcher overrides the
upstream default.

Change-Id: Ic71e5e475d043276d916298ab5acb5c9b9ad063e
Reviewed-on: https://go-review.googlesource.com/45812
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>