Handle errors from backend properly

[igoroshko]

At the moment if backend fails to create an event, the frontend app will still display success message. This should be fixed. We can use same popup for now, but display the error message from the backend.

[suppermancool]

@igoroshko done in topcoder-platform/community-app#6720

[igoroshko]

This can be verified by following steps described in issue #23

[Dara-K]

Makes sense and works, thank you!
Let's close this ticket @igoroshko