Enable the errorAssertions govet check for mattermost-server code (#17346)

* Enable the errorAssertions govet check for mattermost-server code

* Removing unnecesary change

* Fixing some tests

* Fixing tests

* Fixing more after merge

* Fixing new offending entries

* Fixing small vet checks

* Fixing new cases detected by govet

* Fixing remote_cluster_test errors

* Fixing assertion

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/server_test.go

@@ -173,10 +173,10 @@ func TestStartServerNoS3Bucket(t *testing.T) {
 	require.NoError(t, err)
 
 	// ensure that a new bucket was created
-	backend, err := s.FileBackend()
-	require.Nil(t, err)
+	backend, appErr := s.FileBackend()
+	require.Nil(t, appErr)
 	err = backend.(*filestore.S3FileBackend).TestConnection()
-	require.Nil(t, err)
+	require.NoError(t, err)
 }
 
 func TestStartServerTLSSuccess(t *testing.T) {