net/http: add missing error checking reading trailers

This is a simplified version of earlier versions of this CL
and now only fixes obviously incorrect things, without
changing the locking on bodyEOFReader.

I'd like to see if this is sufficient before changing the
locking.

Update #4191

R=golang-dev, rsc, dave
CC=golang-dev
https://golang.org/cl/6739055

src/pkg/net/http/client_test.go

@@ -219,6 +219,9 @@ func TestRedirects(t *testing.T) {
 		return checkErr
 	}}
 	res, err := c.Get(ts.URL)
+	if err != nil {
+		t.Fatalf("Get error: %v", err)
+	}
 	finalUrl := res.Request.URL.String()
 	if e, g := "<nil>", fmt.Sprintf("%v", err); e != g {
 		t.Errorf("with custom client, expected error %q, got %q", e, g)
@@ -335,7 +338,10 @@ func TestRedirectCookiesJar(t *testing.T) {
 	c.Jar = &TestJar{perURL: make(map[string][]*Cookie)}
 	u, _ := url.Parse(ts.URL)
 	c.Jar.SetCookies(u, []*Cookie{expectedCookies[0]})
-	resp, _ := c.Get(ts.URL)
+	resp, err := c.Get(ts.URL)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
 	matchReturnedCookies(t, expectedCookies, resp.Cookies())
 }