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.go

@@ -17,6 +17,7 @@ import (
 	l4g "github.com/alecthomas/log4go"
 	"github.com/gorilla/handlers"
 	"github.com/gorilla/mux"
+	"github.com/pkg/errors"
 	"golang.org/x/crypto/acme/autocert"
 
 	"github.com/mattermost/mattermost-server/model"
@@ -116,7 +117,7 @@ func redirectHTTPToHTTPS(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, url.String(), http.StatusFound)
 }
 
-func (a *App) StartServer() {
+func (a *App) StartServer() error {
 	l4g.Info(utils.T("api.server.start_server.starting.info"))
 
 	var handler http.Handler = &CorsWrapper{a.Config, a.Srv.Router}
@@ -126,8 +127,7 @@ func (a *App) StartServer() {
 
 		rateLimiter, err := NewRateLimiter(&a.Config().RateLimitSettings)
 		if err != nil {
-			l4g.Critical(err.Error())
-			return
+			return err
 		}
 
 		a.Srv.RateLimiter = rateLimiter
@@ -151,8 +151,8 @@ func (a *App) StartServer() {
 
 	listener, err := net.Listen("tcp", addr)
 	if err != nil {
-		l4g.Critical(utils.T("api.server.start_server.starting.critical"), err)
-		return
+		errors.Wrapf(err, utils.T("api.server.start_server.starting.critical"), err)
+		return err
 	}
 	a.Srv.ListenAddr = listener.Addr().(*net.TCPAddr)
 
@@ -214,6 +214,8 @@ func (a *App) StartServer() {
 		}
 		close(a.Srv.didFinishListen)
 	}()
+
+	return nil
 }
 
 type tcpKeepAliveListener struct {