@rstoenescu Thank you very much! However I’ve found a better solution for me right now, and I think that with the new features may be this one can wait a bit and be released forward in a future minor version.
I finally got the point looking at Vue guide reactivity chapter (https://vuejs.org/v2/guide/reactivity.html)
In fact property added after Component initalization are not watched at all.
The best solution is replace the entire error object in that case.