Check if given path exists for core/client openAsX functions

[ndricimrr]

Description

Changes proposed in this pull request:
Fixes #2292

Calls to test with:
Client:

LuigiClient.linkManager().openAsDrawer('badpath', { title: 'a drawer title', size: 'm' });
LuigiClient.linkManager().openAsModal('/badpath', { title: 'a modal title', size: 'm' });
LuigiClient.linkManager().openAsSplitView('/asdadasd', { title: 'splitview title', size: 'm' });

Core:

Luigi.navigation().openAsModal('/gsdsda', { title: 'a modal title', size: 'm' });
Luigi.navigation().openAsSplitView('xcvv', { title: 'a splitview title', size: 'm' });
Luigi.navigation().openAsDrawer('/test123', { title: 'a drawer title', size: 'm' });

Defined handler:
Routing.js

pageNotFoundHandler = (notFounPath, wasAnyPathFitted) => {
    console.log("Wrong path:", notFounPath);
      Luigi.showAlert({
        text: `Path not found: ${wrongPath}`,
        type: 'error',
        closeAfter: 3000
      });
    
    return {redirectTo: "errors"};
  };

with handler defined, should redirect to specified path and show handlers alert.
without handler specified, should show default alert and also print default console warning.

Related issue(s)

[stanleychh]

@ndricimrr , Travis was not happy, Please have a look when you have time 😊