Have meaningful exit codes, not only for cooking

[ilario]

For solving #13, the pull request #15 took exit code from cooking process and returned it at the output of cooker.
The exit code is forced to zero for the other tasks. For example, for downloading the SDK and IB the exit code will be always zero even when a failure happened (related discussion on #12).

[p4u]

I'm thinking about using "set -e" on some parts of the code where an non-zero exit code might be critical.