[Load graph] Display an error message when a json graph file cannot be loaded

Summary

Trying to upload an invalid graph file trigger an endless "loading" message. An error message should be displayed instead.

Steps to reproduce

1. Create a JSON graph file with invalid relation (edge toward a missing node)
2. Try to load the file
3. Observe the endless message..

What is the current bug behavior?

A "loading" message appear, and stay forever.

What is the expected correct behavior?

An error message appears instead.