client: fixes for govet -shadow

This fixes for shadowed error variables found by go tip go tool vet.
Fixes for https://github.com/coreos/etcd/issues/3954.

client/auth_role.go

@@ -115,14 +115,13 @@ func (r *httpAuthRoleAPI) ListRoles(ctx context.Context) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	if err := assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
+	if err = assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
 		return nil, err
 	}
 	var userList struct {
 		Roles []string `json:"roles"`
 	}
-	err = json.Unmarshal(body, &userList)
-	if err != nil {
+	if err = json.Unmarshal(body, &userList); err != nil {
 		return nil, err
 	}
 	return userList.Roles, nil
@@ -218,17 +217,16 @@ func (r *httpAuthRoleAPI) modRole(ctx context.Context, req *authRoleAPIAction) (
 	if err != nil {
 		return nil, err
 	}
-	if err := assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
+	if err = assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
 		var sec authError
-		err := json.Unmarshal(body, &sec)
+		err = json.Unmarshal(body, &sec)
 		if err != nil {
 			return nil, err
 		}
 		return nil, sec
 	}
 	var role Role
-	err = json.Unmarshal(body, &role)
-	if err != nil {
+	if err = json.Unmarshal(body, &role); err != nil {
 		return nil, err
 	}
 	return &role, nil

client/auth_user.go

@@ -78,9 +78,9 @@ func (s *httpAuthAPI) enableDisable(ctx context.Context, req httpAction) error {
 	if err != nil {
 		return err
 	}
-	if err := assertStatusCode(resp.StatusCode, http.StatusOK, http.StatusCreated); err != nil {
+	if err = assertStatusCode(resp.StatusCode, http.StatusOK, http.StatusCreated); err != nil {
 		var sec authError
-		err := json.Unmarshal(body, &sec)
+		err = json.Unmarshal(body, &sec)
 		if err != nil {
 			return err
 		}
@@ -179,9 +179,9 @@ func (u *httpAuthUserAPI) ListUsers(ctx context.Context) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	if err := assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
+	if err = assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
 		var sec authError
-		err := json.Unmarshal(body, &sec)
+		err = json.Unmarshal(body, &sec)
 		if err != nil {
 			return nil, err
 		}
@@ -190,8 +190,7 @@ func (u *httpAuthUserAPI) ListUsers(ctx context.Context) ([]string, error) {
 	var userList struct {
 		Users []string `json:"users"`
 	}
-	err = json.Unmarshal(body, &userList)
-	if err != nil {
+	if err = json.Unmarshal(body, &userList); err != nil {
 		return nil, err
 	}
 	return userList.Users, nil
@@ -221,9 +220,9 @@ func (u *httpAuthUserAPI) addRemoveUser(ctx context.Context, req *authUserAPIAct
 	if err != nil {
 		return err
 	}
-	if err := assertStatusCode(resp.StatusCode, http.StatusOK, http.StatusCreated); err != nil {
+	if err = assertStatusCode(resp.StatusCode, http.StatusOK, http.StatusCreated); err != nil {
 		var sec authError
-		err := json.Unmarshal(body, &sec)
+		err = json.Unmarshal(body, &sec)
 		if err != nil {
 			return err
 		}
@@ -280,17 +279,16 @@ func (u *httpAuthUserAPI) modUser(ctx context.Context, req *authUserAPIAction) (
 	if err != nil {
 		return nil, err
 	}
-	if err := assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
+	if err = assertStatusCode(resp.StatusCode, http.StatusOK); err != nil {
 		var sec authError
-		err := json.Unmarshal(body, &sec)
+		err = json.Unmarshal(body, &sec)
 		if err != nil {
 			return nil, err
 		}
 		return nil, sec
 	}
 	var user User
-	err = json.Unmarshal(body, &user)
-	if err != nil {
+	if err = json.Unmarshal(body, &user); err != nil {
 		return nil, err
 	}
 	return &user, nil