hash_util: Show error if url is invalid.

For urls we cannot handle, we inform user via a nice
error message.

See comment in decodeHashComponent for some of the cases it
fixes for us.

frontend_tests/node_tests/hash_util.js

@@ -1,10 +1,18 @@
-
 zrequire('hash_util');
 zrequire('stream_data');
 zrequire('people');
 zrequire('Filter', 'js/filter');
 zrequire('narrow_state');
 
+set_global('$', global.make_zjquery({
+    silent: true,
+}));
+set_global('ui_report', {
+    displayed_error: false,
+    error: () => {
+        ui_report.displayed_error = true;
+    },
+});
 set_global('location', {
     protocol: "https:",
     host: "example.com",
@@ -60,6 +68,11 @@ run_test('hash_util', () => {
     operand = 'testing 123';
 
     encode_decode_operand(operator, operand, 'testing.20123');
+
+    // Test invalid url decode.
+    const result = hash_util.decodeHashComponent("foo.foo");
+    assert.equal(result, "");
+    assert.equal(ui_report.displayed_error, true);
 });
 
 run_test('test_get_hash_category', () => {

static/js/hash_util.js

@@ -56,7 +56,18 @@ exports.encode_stream_name = function (operand) {
 };
 
 exports.decodeHashComponent = function (str) {
+    try {
+        // This fails for URLS containing
+        // foo.foo or foo%foo due to our fault in special handling
+        // of such characters when encoding. This can also,
+        // fail independent of our fault, so just tell the user
+        // that the url is invalid.
+        // TODO: Show possible valid urls to the user.
         return decodeURIComponent(str.replace(/\./g, '%'));
+    } catch (e) {
+        ui_report.error(i18n.t("Invalid URL"), undefined, $("#home-error"), 2000);
+        return "";
+    }
 };
 
 exports.decode_operand = function (operator, operand) {