etcdhttp: use EcodePrevValueRequired when appropriate

etcdserver/etcdhttp/client.go

@@ -342,7 +342,7 @@ func parseKeyRequest(r *http.Request, id uint64, clock clockwork.Clock) (etcdser
 	pV := r.FormValue("prevValue")
 	if _, ok := r.Form["prevValue"]; ok && pV == "" {
 		return emptyReq, etcdErr.NewRequestError(
-			etcdErr.EcodeInvalidField,
+			etcdErr.EcodePrevValueRequired,
 			`"prevValue" cannot be empty`,
 		)
 	}

etcdserver/etcdhttp/client_test.go

@@ -257,7 +257,7 @@ func TestBadParseRequest(t *testing.T) {
 		// prevValue cannot be empty
 		{
 			mustNewForm(t, "foo", url.Values{"prevValue": []string{""}}),
-			etcdErr.EcodeInvalidField,
+			etcdErr.EcodePrevValueRequired,
 		},
 		// wait is only valid with GET requests
 		{