debug: send back an error response if variable loading fails

[polinasok]

Variable request handler should detect if variable auto-loading via expression evaluation in loadChildren (e.g. like in #135, #1052) failed and return an error response in that case. Currently the error is only logged to the DEBUG CONSOLE and is easy to miss, especially if verbose logging is on. If we return the error via the response, it will be displayed in the VARIABLES pane, and the option to expand for more children (which would only return more errors) would be disabled.

[polinasok]

dlv-dap is taking a different approach here for reporting errors:

Instead of returning a failed request, it prints the type of the variable along with the error and disables loading of any children.

[polinasok]

For types where we do not have auto-loading yet, we report things like so: