Mapbuilder frontend errors

[ajuvercr]

Waiting for #8

When you create a new map, things will fail. Display the errors, in some way.

It would be nice to have a uniform way of displaying errors, like a js function in backend/templates/base.html.tera or something.
But displaying the errors in a div in an other way is perfect too!

Useful files: backend/templates/mapbuilder.html.tera, backend/static/style/{mapbuilder, style}.css, backend/static/script/mapbuilder.js.