net/http: remove some log spam in test, add missing error detail

Updates #22540

Change-Id: I26e79c25652976fac6f2e5a7afb4fd1240996d74
Reviewed-on: https://go-review.googlesource.com/75531
Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport_test.go

@@ -1113,15 +1113,14 @@ func TestTransportProxy(t *testing.T) {
 						Header:     make(Header),
 					}
 
-					log.Printf("Dialing %s", r.URL.Host)
 					targetConn, err := net.Dial("tcp", r.URL.Host)
 					if err != nil {
-						t.Errorf("net.Dial failed")
+						t.Errorf("net.Dial(%q) failed: %v", r.URL.Host, err)
 						return
 					}
 
 					if err := res.Write(clientConn); err != nil {
-						t.Errorf("Writing 200 OK failed")
+						t.Errorf("Writing 200 OK failed: %v", err)
 						return
 					}