cmd/pprof: remove redundant URLs in error messages

[vcabbage]

What version of Go are you using (go version)?

go version go1.8rc2 darwin/amd64

What did you do?

As observed in #18746:

# go tool pprof ./project http://localhost:5000/debug/pprof/profile
Fetching profile from http://localhost:5000/debug/pprof/profile
Please wait... (30s)
http fetch http://localhost:5000/debug/pprof/profile: Get http://localhost:5000/debug/pprof/profile: EOF

The URLs in the message are redundant, making the relevant portion less obvious. I propose omitting the redundant URL when the error is of type *url.Error.

[bradfitz]

It's always of type *url.Error if it comes from an http.Client.

[gopherbot]

CL https://golang.org/cl/35650 mentions this issue.