Show remediation steps and human readable error for `Cannot change source virtual machine` error

If an already existing managed image is attempted to be modified with a new VM, this can lead to a confusing error "cannot change source virtual machine", users often get confused and think this is a plugin bug, we can catch this specific error and return a useful message like

```
Failed to create Managed Image: Cannot change source virtual machine

This error usually happens when running two parallel builds simultaneously against the name `managed_image_name`, leading to the images conflicting when one build tries to update the source VM of the other
```

Start by locating the Go code that handles creation or modification of Azure managed images and search for the exact “cannot change source virtual machine” error. Confirm how this conflict is surfaced, then verify that the completed behavior presents the requested human-readable explanation and parallel-build remediation steps.

Written by the indexing model from the issue text.