#85645 display Variables fetch failure message

[gjsjohnmurray]

This PR fixes #85645.

A modified vscode-mock-debug is available at https://github.com/gjsjohnmurray/vscode-mock-debug/tree/show-85645 in which the Variables request always returns a failure status and a message.

Prior to this PR the message was not displayed in the Variables panel:

After the fix:

[isidorn]

Thanks for this PR. Even though PR indeed fixes the issue the actual probel is the following:
Variables that have available = false are not properly rendered in the VARIABLES view. I am not sure why is that. The idea behind available = false is that we can render the variable like an error.

So ideally we would leave for that variable availble = false and we would fix the rendering
If that is not possible, than I am fine that we remove the available attribute altogether (might be a cleaner solution and I do not mind that we loose the special rendering)

Let me know what you think and thanks a lot!

[gjsjohnmurray]

@isidorn here's what it now gives. Is that what you meant?

[isidorn]

Looks good, tahnks a lot for the PR!