[network] Add an error_ctx field in variable_value message

It will allow to retrieve more information / context from failure
[model] Make updater able to return its error_ctx
[model] Add an error() in compute_expression

This error, in particular, allow to retrieve errors related to failed to
get remote variable values
[model] Instruct more subsystem about error_context
[compiler] Properly generate code to handle failed_read error ...
[model] Add some helper to display full error_context
[model] Teach execution part about error_context

While here, arrange place calling it.
[compiler] Add a separate counter for abortable_computation

Previously, in some situation, it is possible to generated malformed
code, which create more than once variables with the same name.