[model] Make error_context go back until the caller

Add some global error_context in logic_ctx, and fill it with the content
of task error_context.
Pass the error_context in the final callback and the update_status
Fill properly the err_ctx field of constraint_answer_msg
[model] Cosmetic changes in tasks

- Use network::error_context instead of its explicit definition
- rename error_context in err_ctx to match other code place
- make them private as they must be only modified by task.cc
[model] Add some possible cause to runtime_failure and fill it

It will provide to the operator a more clear understanding of what
fails.
[network] Add a pretty print method to runtime_failure