Fail vs. fatal on store startup (#24170)

If the store fails to initialize (e.g. run a migration), it would `log.Fatal` and then `os.Exit`. Unfortunately, this trips up `TestMain`, which happily keeps running tests, now guaranteed to fail.

Avoid this by instead returning an error from the store initialization, handling appropriately at the layer above.

server/channels/store/sqlstore/migrate_test.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestUpAndDownMigrations(t *testing.T) {
@@ -23,7 +24,8 @@ func TestUpAndDownMigrations(t *testing.T) {
 				t.Skip(err)
 			}
 
-			store := New(*settings, nil)
+			store, err := New(*settings, nil)
+			require.NoError(t, err)
 			defer store.Close()
 
 			err = store.migrate(migrationsDirectionDown, false)