improve logging

[manut]

introduce loglevels, abort built on critical erros.

[manut]

needed for #144

[manut]

Hello all,

I have once again wasted some time to find out that the build
did not complete successfully (error in finetune block) by
wondering why things did not work as expected.

In this particular case:
openssh-server is installed
finetune is editing a line in /etc/ssh/sshd_config --- but
failed, since the file was not there.

Needless to say, I went all the way installing the image
on the target and wondering why I could not log in ...

So I would like to make the case again, that the build
should abort on the slightest error, at least in the
finetune block, but maybe everywhere UNLESS explicitly
stated otherwise (think: cmd || true # this cmd may fail)

Cheers,
Erich

[manut]

return useful errorcodes

[manut]

there is a long cap between

"100% update finished"

and the next log output. This makes some users believe the elbe build process inside the initvm died.