Abort on critical error during server startup (#8204)

Only a handful of critical errors are present in the codebase.
They all occur during server startup (in `app.StartServer()`).

Currently, when one of these critical error occurs, it is simpled
mentionned in the logs – then the error is discarded, and the app
attempts to continue the execution (and probably fails pretty quickly in
a weird way).

Rather than continuing operations in an unknow state, these errors should
trigger a clean exit.

This commit rewrites critical startup errors to be correctly
propagated, logged, and then terminate the command execution.
Additionnaly, it makes the server return a proper error code to the
shell.

app/server_test.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"testing"
+
+	"github.com/mattermost/mattermost-server/model"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStartServerSuccess(t *testing.T) {
+	a, err := New()
+	require.NoError(t, err)
+
+	a.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.ListenAddress = ":0" })
+	serverErr := a.StartServer()
+	a.Shutdown()
+	require.NoError(t, serverErr)
+}
+
+func TestStartServerRateLimiterCriticalError(t *testing.T) {
+	a, err := New()
+	require.NoError(t, err)
+
+	// Attempt to use Rate Limiter with an invalid config
+	a.UpdateConfig(func(cfg *model.Config) {
+		*cfg.RateLimitSettings.Enable   = true
+		*cfg.RateLimitSettings.MaxBurst = -100
+	})
+
+	serverErr := a.StartServer()
+	a.Shutdown()
+	require.Error(t, serverErr)
+}
+
+func TestStartServerPortUnavailable(t *testing.T) {
+	a, err := New()
+	require.NoError(t, err)
+
+	// Attempt to listen on a system-reserved port
+	a.UpdateConfig(func(cfg *model.Config) {
+		*cfg.ServiceSettings.ListenAddress = ":21"
+	})
+
+	serverErr := a.StartServer()
+	a.Shutdown()
+	require.Error(t, serverErr)
+}