cmd/go/internal/modfetch: wrap errors from HTTP response bodies
The Read method on the Body returned from a net/http.Transport does
not wrap errors returned by the underlying io.Reader and returns a
bare io.ErrUnexpectedEOF if the body is shorter than the declared
Content-Length.
Since we can't feasibly add detail on the net/http side without
breaking established users, we must instead add detail on the caller
side. Since the net/http client uses url.Error for most of its own
errors, we use that same error type here.
I have not added a regression test for this change. (While it is
theoretically possible to set up a GOPROXY that returns incorrect
Content-Length headers, the change seems straightforward enough that
it isn't worth the complex test setup.)