Rebasing MR fails silently if there are conflicts

Summary

Rebasing MR fails silently if there are conflicts

Steps to reproduce

Create a MR that is behind target branch and would have conflicts when rebasing. Try to use the Web UI "Rebase" button.

What is the current bug behavior?

No message, UI gets back to previous state with no explanation

What is the expected correct behavior?

Information about conflicts is displayed.

Either:

• the conflicts can be resolved via Web UI
• steps to resolve conflicts in CLI are displayed

Relevant logs and/or screenshots