fix(error-display): component remains displayed after player reset (#8482)

When `player.reset` is called, the `errorDisplay` component is not reset, and neither is `player.error`.

- Sets `player.error` to `null`, so that the `player.errorDisplay` and `player.error` are correctly reset.
- Adds an `error` function to the `testPlayer` stub to prevent tests from failing.

src/js/player.js

@@ -3604,6 +3604,9 @@ class Player extends Component {
     this.loadTech_(this.options_.techOrder[0], null);
     this.techCall_('reset');
     this.resetControlBarUI_();
+
+    this.error(null);
+
     if (isEvented(this)) {
       this.trigger('playerreset');
     }

test/unit/player.test.js

@@ -1910,6 +1910,7 @@ QUnit.test('player#reset loads the Html5 tech and then techCalls reset', functio
     options_: {
       techOrder: ['html5', 'youtube']
     },
+    error() {},
     resetCache_() {},
     loadTech_(tech, source) {
       loadedTech = tech;
@@ -1942,6 +1943,7 @@ QUnit.test('player#reset loads the first item in the techOrder and then techCall
     options_: {
       techOrder: ['youtube', 'html5']
     },
+    error() {},
     resetCache_() {},
     loadTech_(tech, source) {
       loadedTech = tech;

test/unit/reset-ui.test.js

@@ -144,3 +144,19 @@ QUnit.test('Calling resetVolumeBar player method should reset volume bar', funct
 
   player.dispose();
 });
+
+QUnit.test('Calling reset player method should reset both error display and player error', function(assert) {
+  const player = TestHelpers.makePlayer({techOrder: ['html5']});
+
+  player.error('ERROR');
+
+  assert.notOk(player.errorDisplay.hasClass('vjs-hidden'), 'ErrorDisplay is displayed if there is an error');
+  assert.strictEqual(player.error().message, 'ERROR', 'player error has content');
+
+  player.reset();
+
+  assert.ok(player.errorDisplay.hasClass('vjs-hidden'), 'ErrorDisplay is not displayed if there is no error');
+  assert.strictEqual(player.error(), null, 'player error has content');
+
+  player.dispose();
+});