Improve logging in the builder

Raise one message from debug to info and add the name of the service in
definition.json so we can use it for logging.
Add utils.strsignal to convert between a signal number and name

Convert signum to e.g: SIGINT. Not exactly like the actual strsignal
function but close enough.
Catch subprocess.CalledProcessError exceptions in the builder

And output human friendly error messages.
Move ignore_eexist into utils
Move the supervisor configuration generation into the builder

This makes more sense and will be needed to correctly support the custom
service anyway.

Closes #1.
Symlink current from the Service class

This is needed to properly customize the build for the custom service
(which doesn't create a current symlink).
Add env support in Container.run

So we can run the build with HOME set which is mandatory for login shell
to work correctly.
Run the build with HOME set, so login shells can work
Add support for the custom service