MM-51786: Sentry crash while installing a blocked plugin (#22804)

We would return "nil, nil" if a plugin was on the blocklist.
This would cause a nil dereference panic while trying to add
struct fields from manifest to the audit logs.

To avoid it, we return an error explicitly and ignore the
error ids to unnecessarily have it log an error.

https://mattermost.atlassian.net/browse/MM-51786

```release-note
NONE
```

server/channels/app/plugin_install_test.go

@@ -172,10 +172,9 @@ func TestInstallPluginLocally(t *testing.T) {
 		defer th.TearDown()
 		cleanExistingBundles(t, th)
 
-		manifest, appErr := installPlugin(t, th, "playbooks", "0.0.1", installPluginLocallyAlways)
-		require.Nil(t, appErr)
-		require.Nil(t, manifest)
-
+		_, appErr := installPlugin(t, th, "playbooks", "0.0.1", installPluginLocallyAlways)
+		require.NotNil(t, appErr)
+		require.Equal(t, "app.plugin.blocked.app_error", appErr.Id)
 		assertBundleInfoManifests(t, th, []*model.Manifest{})
 	})
 
@@ -222,9 +221,9 @@ func TestInstallPluginLocally(t *testing.T) {
 			require.Nil(t, appErr)
 			require.NotNil(t, existingManifest)
 
-			manifest, appErr := installPlugin(t, th, "valid", "0.0.1", installPluginLocallyOnlyIfNewOrUpgrade)
-			require.Nil(t, appErr)
-			require.Nil(t, manifest)
+			_, appErr = installPlugin(t, th, "valid", "0.0.1", installPluginLocallyOnlyIfNewOrUpgrade)
+			require.NotNil(t, appErr)
+			require.Equal(t, "app.plugin.skip_installation.app_error", appErr.Id)
 
 			assertBundleInfoManifests(t, th, []*model.Manifest{existingManifest})
 		})
@@ -238,9 +237,9 @@ func TestInstallPluginLocally(t *testing.T) {
 			require.Nil(t, appErr)
 			require.NotNil(t, existingManifest)
 
-			manifest, appErr := installPlugin(t, th, "valid", "0.0.2", installPluginLocallyOnlyIfNewOrUpgrade)
-			require.Nil(t, appErr)
-			require.Nil(t, manifest)
+			_, appErr = installPlugin(t, th, "valid", "0.0.2", installPluginLocallyOnlyIfNewOrUpgrade)
+			require.NotNil(t, appErr)
+			require.Equal(t, "app.plugin.skip_installation.app_error", appErr.Id)
 
 			assertBundleInfoManifests(t, th, []*model.Manifest{existingManifest})
 		})