Display validation errors (if any) during model submission

[Camille Laibe]

In case of validation issues detected during submission, their details should be displayed to the user, providing enough information to allow him/her to fix and re-submit successfully the model.

[Raza Ali]

Resolved: https://bitbucket.org/jummp/jummp/commits/b44ae5d9cc81538a274092db39cb00b2a5e97787