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/app_test.go

@@ -51,7 +51,8 @@ func TestAppRace(t *testing.T) {
 		a, err := New()
 		require.NoError(t, err)
 		a.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.ListenAddress = ":0" })
-		a.StartServer()
+		serverErr := a.StartServer()
+		require.NoError(t, serverErr)
 		a.Shutdown()
 	}
 }