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.

web/web_test.go

@@ -44,7 +44,10 @@ func Setup() *app.App {
 	}
 	prevListenAddress := *a.Config().ServiceSettings.ListenAddress
 	a.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.ListenAddress = ":0" })
-	a.StartServer()
+	serverErr := a.StartServer()
+	if serverErr != nil {
+		panic(serverErr)
+	}
 	a.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.ListenAddress = prevListenAddress })
 	api4.Init(a, a.Srv.Router, false)
 	api3 := api.Init(a, a.Srv.Router)