I found that any exception raised from generator ends with StopIteration
at next next() call. So, the solution wasn't able to provide
bulletproof solution for unexpected errors.

This solution uses some other nice-to-look-at shortcut to not duplicate
code that is based on closures.

I'm able to protect in-out boxes against unexpected errors. I'm not
able to do the same for out-only boxes based on generators, but it is
better than nothing.

Boxes generating messages should know what they are doing, so they
shouldn't need so much effort.

TODO: Will be nice to transform it into variable with error code.

The LoopFail exception will by eaten by except Exception, but I want to
be able to distinguish between broken input and broken output error in
logs.

I need to re-raise it otherwise it will be eaten by except Exception.

Else branch for generator shouldn't be inside "main" while. It ignores
StopIteration and re-run generator again and again.

I use the same hack as in sn.SN itself.

It doesn't work with mock from some reason.

I will need to generate some resources in env_data that has
informations in sn_main function and I don't want to need to pass them
to _sn_main_loop. This could be the way to avoid it.