net/url: report first error from ParseQuery.

Fixes #4175.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6610068

src/pkg/net/url/url.go

@@ -521,12 +521,16 @@ func parseQuery(m Values, query string) (err error) {
 		}
 		key, err1 := QueryUnescape(key)
 		if err1 != nil {
-			err = err1
+			if err == nil {
+				err = err1
+			}
 			continue
 		}
 		value, err1 = QueryUnescape(value)
 		if err1 != nil {
-			err = err1
+			if err == nil {
+				err = err1
+			}
 			continue
 		}
 		m[key] = append(m[key], value)

src/pkg/net/url/url_test.go

@@ -7,6 +7,7 @@ package url
 import (
 	"fmt"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -779,3 +780,13 @@ func TestRequestURI(t *testing.T) {
 		}
 	}
 }
+
+func TestParseFailure(t *testing.T) {
+	// Test that the first parse error is returned.
+	const url = "%gh&%ij"
+	_, err := ParseQuery(url)
+	errStr := fmt.Sprint(err)
+	if !strings.Contains(errStr, "%gh") {
+		t.Errorf(`ParseQuery(%q) returned error %q, want something containing %q"`, url, errStr, "%gh")
+	}
+}