Add support for form errors

[rabauss]

This is a draft to solve #258 It depends on https://github.com/contao/contao/pull/4898

At the moment I added form errors to the exception catches. Maybe we could add some more errors. Currently I did not add any special error messages, it will always be ERR.general but could be overriden.

[Toflar]

I honestly appreciate the work but there won't be any 1.x compatible with Contao 5 so the implementation would be completely different anyway, I guess :)

[rabauss]

Yes, I've made a patch for 4.13 of my core changes. But at the moment I cannot install the latest notification center in my project because of incompatibility with contao-haste 4 😞

[Toflar]

😊 I'm closing this then. We can talk about general form errors again in a future 2.x version.